usb: musb: Disable interrupts on suspend, enable them on resume

In certain situations, an interrupt triggers on resume, before musb_start()
has been called. This has been observed to cause enumeration issues after
suspend/resume cycles with AM335x.

Signed-off-by: Pascal Huerst <pascal.huerst@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>