msm_serial_hs: Remove code dependency on flag tty_flush_receive

Removing code dependency on the flag and check the
circular buffer directly and make sure that spin lock is taken
appropriately in the tx lower half.

This avoids the condition that can cause
1. tx lower half
2. submit tx command
to race.

CRs-Fixed: 621210
Change-Id: I344a72c3eb9770391576fa0e8f0a8da1ee50ea5b
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>