msm_serial_hs: Fix wakeup byte data type and log

Wakeup byte should be an unsigned char to match the
definition of tty_insert_flip_string where it is used
and add debug level log for the same.

Change-Id: I6f558d68e7fb99ffaf5c287e703576c2340e5bcb
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>