msm_serial_hs: disable TX and RX during baudrate change

Disable TX and RX during baudrate change function such that
hardware can resume operation in the new setting. After the
issue of the baudrate change request, the data in the pipe
is to be discarded and thus this doesn't result in data loss
for the clients.

Change-Id: Iad729e442518fe92410ae0444cf8565e42db0074
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>