tty: serial: msm_geni_serial: Handle the Console RX overflow

Debug UART does not have flow control lines and that can lead to an
overflow in the RX direction. Keep the RX FIFO watermark to a small
value so that the UART can provide a quicker response and minimize
the overflow. In case of an overflow, update the right statistics that
can be monitored through proc file system. Also insert the TTY_OVERRUN
character to the framework to indicate overflow scenario.

Change-Id: I0b6a6cadcd76a85ac0130efbc7d5b94d9bdbad17
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>