msm_serial_hs: modify uart driver to prevent race between timer calls

In some of the cases when UART submits sps transfer request, BAM finishes
transfer quickly. This results into deletion of Tx timeout timer from an
ISR context whereas in actual timer is not yet started. In such cases
del_timer and mod_timer are meeting to a race where mod_timer finds a
NULL handle due to deletion by del_timer.

Change the timer call sequence to submit the timer first and then do a
BAM transfer.

Change-Id: I1d75a43d928776eb24752fd97d0ef31e0d5b6a51
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>