staging/rtl8192u: fix defined but not used build warnings

Fix build warnings in rtl8192u when CONFIG_PROC_FS is not enabled
by marking the unused functions as __maybe_unused.

../drivers/staging/rtl8192u/r8192U_core.c:508:12: warning: 'proc_get_stats_ap' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:527:12: warning: 'proc_get_registers' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:568:12: warning: 'proc_get_stats_tx' defined but not used [-Wunused-function]
../drivers/staging/rtl8192u/r8192U_core.c:627:12: warning: 'proc_get_stats_rx' defined but not used [-Wunused-function]

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Jerry chuang <wlanfae@realtek.com>
Cc: devel@driverdev.osuosl.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/rtl8192u/r8192U.h

@@ -18,6 +18,7 @@
 #ifndef R819xU_H
 #define R819xU_H
 
+#include <linux/compiler.h>
 #include <linux/module.h>
 #include <linux/kernel.h>
 #include <linux/ioport.h>

drivers/staging/rtl8192u/r8192U_core.c

@@ -505,7 +505,7 @@ static void watch_dog_timer_callback(struct timer_list *t);
 
 static struct proc_dir_entry *rtl8192_proc;
 
-static int proc_get_stats_ap(struct seq_file *m, void *v)
+static int __maybe_unused proc_get_stats_ap(struct seq_file *m, void *v)
 {
 	struct net_device *dev = m->private;
 	struct r8192_priv *priv = (struct r8192_priv *)ieee80211_priv(dev);
@@ -524,7 +524,7 @@ static int proc_get_stats_ap(struct seq_file *m, void *v)
 	return 0;
 }
 
-static int proc_get_registers(struct seq_file *m, void *v)
+static int __maybe_unused proc_get_registers(struct seq_file *m, void *v)
 {
 	struct net_device *dev = m->private;
 	int i, n, max = 0xff;
@@ -565,7 +565,7 @@ static int proc_get_registers(struct seq_file *m, void *v)
 	return 0;
 }
 
-static int proc_get_stats_tx(struct seq_file *m, void *v)
+static int __maybe_unused proc_get_stats_tx(struct seq_file *m, void *v)
 {
 	struct net_device *dev = m->private;
 	struct r8192_priv *priv = (struct r8192_priv *)ieee80211_priv(dev);
@@ -624,7 +624,7 @@ static int proc_get_stats_tx(struct seq_file *m, void *v)
 	return 0;
 }
 
-static int proc_get_stats_rx(struct seq_file *m, void *v)
+static int __maybe_unused proc_get_stats_rx(struct seq_file *m, void *v)
 {
 	struct net_device *dev = m->private;
 	struct r8192_priv *priv = (struct r8192_priv *)ieee80211_priv(dev);