usb: musb: Allow running without CONFIG_USB_PHY

Modify the core so that musb->xceiv is never deferenced without being
checked first.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20221026182657.146630-4-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>