phy: core: make NULL a valid phy reference if !CONFIG_GENERIC_PHY

This fixes a regression on Keystone 2 platforms caused by patch
57303488cd37da58263e842de134dc65f7c626d5
"usb: dwc3: adapt dwc3 core to use Generic PHY Framework" which adds
optional support of generic phy in DWC3 core.

On Keystone 2 platforms the USB is not working now because
CONFIG_GENERIC_PHY isn't set and, as result, Generic PHY APIs stubs
return -ENOSYS always. The log shows:
 dwc3 2690000.dwc3: failed to initialize core
 dwc3: probe of 2690000.dwc3 failed with error -38

Hence, fix it by making NULL a valid phy reference in Generic PHY
APIs stubs in the same way as it was done by the patch
04c2facad8fee66c981a51852806d8923336f362 "drivers: phy: Make NULL
a valid phy reference".

Acked-by: Felipe Balbi <balbi@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/phy/phy.h

@@ -174,21 +174,29 @@ void devm_of_phy_provider_unregister(struct device *dev,
 #else
 static inline int phy_pm_runtime_get(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_pm_runtime_get_sync(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_pm_runtime_put(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_pm_runtime_put_sync(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
@@ -204,21 +212,29 @@ static inline void phy_pm_runtime_forbid(struct phy *phy)
 
 static inline int phy_init(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_exit(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_power_on(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }
 
 static inline int phy_power_off(struct phy *phy)
 {
+	if (!phy)
+		return 0;
 	return -ENOSYS;
 }