msm_serial_hs: consider flush buffer request only when DMA is in progress

TTY Core uses flush_buffer to clear out any output data before closing
port, whereas UART application can always request to do flush output data
before starting data transfer using tty supported ioctl().
receive_tty_flush is used to make decision on completion of any DMA Tx
data transfer. Using same approach when there is no DMA Tx transfer is
happening, it is breaking next DMA Tx transfer functionality. Hence
update receive_tty_flush only when there is any DMA TX transfer active.

CRs-Fixed: 438057
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
(cherry picked from commit 53069734628ebce341cf1ef778c91dbcbbcf67db)

Change-Id: I46cc5cac14d072a8dd8fb5bc76a24a7653f1a7b6
Signed-off-by: Sivasri Kumar Vanka <sivasri@codeaurora.org>