serial: msm_geni_serial: Reduce wait for transfer delay

During port_close wait_for_transfers_inflight() function is checking
for DMA_2_RX_LENGTH_IN register in dma mode. This register counts
the number of bytes written to system memory. Its value is valid
until new transfer is issued. In dma mode wait_for_transfers_inflight()
function is iterating in a loop to read DMA_2_RX_LENGTH_IN register for
50 times with 10msec delay which is introducing total 500msec delay
during port_close()/BT off usecase.

Hence reduce WAIT_XFER_MAX_ITER counter value to reduce the delay.

Change-Id: Iad862178ca603a6632b3e0551269687b23499602
Signed-off-by: Chandana Kishori Chiluveru <cchiluve@codeaurora.org>