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

This fixes a regression on Keystone 2 platforms caused by patch
57303488
"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
04c2faca "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>