msm: mhi_dev: Handle multiple RS232 control signals

Host may send multiple RS232 signals (commands) in one shot. Add
code to handle all commands and notify client(s).

Change-Id: I5369c2206745f07a880718e4afde08e3fea004b4
Signed-off-by: Siva Kumar Akkireddi <sivaa@codeaurora.org>