serial: msm_geni_serial: Fix UART hang

If a serial console write occurred while a UART transmit command was
waiting for a done signal then no further data would be sent until
something new kicked the system into gear. If there is already data
waiting in the circular buffer we must re-enable the tx watermark so we
receive the expected interrupts.

(am from https://git.kernel.org/pub/scm/linux/kernel/git/next/linux-


 next.git/commit/drivers/tty/serial/qcom_geni_serial.c?h=next-20190716&id=
 663abb1a7a7ff8fea9ab0145463de7fcff823755).

Change-Id: Ibef3a7e2fca72f2356d5d43845056cefb6b0d2a4
Signed-off-by: Mitul Golani <mgolani@codeaurora.org>
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>