usb: gadget: function: f_hid: fix sparse warning

this patch fixes following sparse warning:
f_hid.c:572:30: warning: symbol 'f_hidg_fops' was not declared. Should it be static?

Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

drivers/usb/gadget/function/f_hid.c

@@ -569,7 +569,7 @@ fail:
 	return status;
 }
 
-const struct file_operations f_hidg_fops = {
+static const struct file_operations f_hidg_fops = {
 	.owner		= THIS_MODULE,
 	.open		= f_hidg_open,
 	.release	= f_hidg_release,