usb: core: phy: make it a no-op if CONFIG_GENERIC_PHY is disabled

If the generic PHY support is disabled the stub of devm_of_phy_get_by_index
returns ENOSYS. This corner case isn't handled properly by
usb_phy_roothub_add_phy and at least breaks USB support on Raspberry Pi
(bcm2835_defconfig):

    dwc2 20980000.usb: dwc2_hcd_init() FAILED, returning -38
    dwc2: probe of 20980000.usb failed with error -38

Let usb_phy_roothub_alloc() return in case CONFIG_GENERIC_PHY is
disabled to fix this issue (compilers might even be smart enough to
optimize away most of the code within usb_phy_roothub_alloc and
usb_phy_roothub_add_phy if CONFIG_GENERIC_PHY is disabled). All
existing usb_phy_roothub_* functions are already NULL-safe, so no
special handling is required there.

Fixes: 07dbff0d ("usb: core: add a wrapper for the USB PHYs on the HCD")
Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>