msm_serial_hs: make the Bluetooth tty thread RT

For Bluetooth, the tty kthread should be RT in order to avoid scheduling
delays.

Cherry picked from:
commit 4c7f1a012302 ("msm_serial_hs: make the Bluetooth tty thread RT")

Bug: 36106419
Test: Play Bluetooth audio, verify tty kthread runs at RT via systrace
Change-Id: Ic14cd4114084230fbbf8eb38dadca4bc248db952
Signed-off-by: Philip Cuadra <philipcuadra@google.com>
Signed-off-by: Petri Gynther <pgynther@google.com>