msm_serial_hs: fix __pm calls so that they are balanced

__pm_stay_awake/__pm_relax calls need to be balanced and hence embed
them in the runtime pm callbacks. When client has voted for clocks
fail system suspend.

Change-Id: Ib6318441b5cef20d78084d1a911d17dd03285dae
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>