USB: gadget: android: mass_storage: Use spin_lock_irqsave/spin_unlock_irqrestore

The old code did not allways disable interrupts when called from thread
context, but tried to lock the same spinlock from interrupt context.

This was merged from a change to drivers/usb/function/mass_storage.c
in the android-msm-2.6.29 branch.

Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: Mike Lockwood <lockwood@android.com>