msm_serial_hs: Initialize tty_flush_recieve while Port Open

Initialize the tty_flush_recieve to default value during
Port Open to avoid a corner case where Tx bytes may get queued
twice by UART in the absence of initialization.

Corner Case:
UART client calls uart_flush_buffer() API to flush the
data on Tx while UART Tx is in middle of operation.
If client immediately calls UART Port close followed by UART Tx
after Port-Open, it may result in queueing same bytes twice

CRs-Fixed: 590459
Change-Id: Ied9d33265060e1fde7fc98e29f58d3eff6ddc3c4
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>