msm_serial_hs: wait for clock-off to complete before shutdown

Make sure clock-off request completes before closing the port to
avoid unclocked hardware access.

CRs-Fixed: 648827
Change-Id: I953a06c104830c5515551d5edc4dd9c9a037203f
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>