serial: msm_geni_serial: Remove interrupt storm

Disable M_TX_FIFO_WATERMARK_EN after we've sent all data for a given
transaction so we don't continue to receive a flurry of free space
interrupts while waiting for the M_CMD_DONE notification. Re-enable the
watermark when establishing the next transaction.

Also clear the watermark interrupt after filling the FIFO so we do not
receive notification again prior to actually having free space.

(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=64a428077758383518c258641e81d57fcd454792).

Change-Id: I6375748dfa4ff0f159874e74abffaf5db180be39
Signed-off-by: Mitul Golani <mgolani@codeaurora.org>
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>