msm_serial_hs: disable interrupts cleanly in shutdown

Clear and disable the interrupts when the port is closed
such that the interrupt state and the flags are consistent
and eliminate potential race condition.

Change-Id: I25310b30e236a990460127eddb1c3b4e7ade1550
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>