msm_serial_hs: Programing the TX FIFO watermark level to 4

As per the Hardware Data Book recommendation the
TX FIFO watermark level should not be zero.
Hence programing the TX FIFO watermark level to
4 to avoid corner cases

CRs-Fixed: 621214
Change-Id: Icee051bd37c389f31a010b51ffaf5d46854306b5
Signed-off-by: Kiran Gunda <kgunda@codeaurora.org>
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>