msm_serial_hs: Fix race condition with BAM rx pipe

Crash observed due to the race condition with bam
rx pipe during sps_disconnect and sps_transfer_one
operations executed at the same time on two CPUs.

Because of this race condition rx bam descriptor
pointer becomes NULL and accessing of this pointer
leads to crash.

This patch waits for sps_transfer_one function to
complete, if it is in progress, before calling
sps_disconnect function from the set_termios.

CRs-Fixed: 534029
Change-Id: I7581062a17a3dc7ede8eb58e4d25e3b517812ab2
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>