tty: serial: msm_geni_serial: Update framework fifo after draining

Currently the tail pointer in the UART framework's circular buffer gets
updated everytime data is transmitted. This enables the framework to
constantly fill the circular buffer if large amount of data needs to be
transmitted. This leads to starvation when there is continuous transmission
at a lesser baud rate.

Update the framework's tail pointer when the entire circular buffer is
drained. This will provide a soft flow control and avoid any starvation.

Change-Id: Icc1fd7071e94917d35bb0edd30e35b14030a648c
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>