ENGR00220341-2 usb: add spin_lock_irqsave protect for pdata->lowpower

ENGR00220341-2 usb: add spin_lock_irqsave protect for pdata->lowpower

pdata->lowpower may be accessed at two drivers together, assumed
the situation that host/device set phy to low power mode but
still not set the flag lowpower, at this time the wakeup occurs, as
the flag lowpower is still not set, the interrupt will be infinite loop
as no one will serve it.

This commit is for driver code and add protect at driver.

Signed-off-by: Peter Chen <peter.chen@freescale.com>