USB: musb: blackfin: pm: make it work

Split the USB MMR init steps out into a helper func that both the platform
init and the resume code may call.

Then while suspending, the gpio_vrsel will change from high to low which
will generate a wakeup event and resume the system immediately, so we need
to manually drive it low before we sleep.

Signed-off-by: Bob Liu <lliubbo@gmail.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>