usb: phy: don't return with NULL from devm_usb_get_phy()

The callers are expecting an ERR_PTR value in case
of an error. Change he code to return with an encoded
-ENOMEM value in the case of a failed devres_alloc call.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>