msm_serial_hs: Remove extra irq enable/disable

IRQs are disabled/enabled twice in the clock off and
clock on operations. This patch removes the extra irq
operations.

Change-Id: I4325bfe8792b1f97f232ac26c17ab17c4ddffa6e
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>