msm_serial_hs: Rx discard flush timeout

While configuring the uart buadrate using set_termios
function, timeout is occuring for rx discard flush
operation. This is due to the driver is waiting for
less time for the DMA call back to occur after programing
the DMA register with the discard flush command.

To overcome this issue increased the timeout to 300ms.

Change-Id: I60db1f9eea8bb59fab4024e5f5dfcbcdcde253bb
Signed-off-by: Dilip Kota <c_dkota@codeaurora.org>