tty: serial: msm: Add timeout for waiting TX ready

At worst case, msm serial driver is stuck in waiting
TX ready, so have a 500ms timeout to avoid stuck
and only lost some log to uart.

Change-Id: I2984ed8ad094a040c8ae49ee6ffba5f0238ca336
Signed-off-by: Maria Yu <aiquny@codeaurora.org>
Signed-off-by: Teng Fei Fan <tengfei@codeaurora.org>