usb: musb: Add and use inline functions musb_{get,set}_state

Instead of manipulating musb->xceiv->otg->state directly, use the newly
introduced musb_get_state() and musb_set_state() inline functions.

Later, these inline functions will be modified to get rid of the
musb->xceiv dependency, which prevents the musb code from using the
generic PHY subsystem.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Link: https://lore.kernel.org/r/20221026182657.146630-2-paul@crapouillou.net
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>