msm_serial: Use spin_lock_irqsave() and spin_lock_irqrestore() apis

Currently spin_lock() and spin_unlock() apis are used while handling
interrupts for synchronization which may create deadlock or schedule
out current interrupt processing part if any other interrupt comes on
the same cpu. Hence use spin_lock_irqsave() and spin_lock_irqrestore()
apis to make driver smp-safe.

Signed-off-by: Mayank Rana <mrana@codeaurora.org>