serial: msm_serial_hs: Modify wakeup handler disablement and processing

Make sure to disable wakeup irq synchronously as in an aggressive
scheduling system if it's asynchronous then unserved interrupt can
get scheduled later. In addition remove the threaded handler and
configure as an ISR to not have scheduler impact on functionalities.

Change-Id: Id525d5295834b73c2f42d52f0e363f0bdec1d22a
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>