tty: serial: qcom_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.

Change-Id: I31a2f495328fb7e67bc7e8eb112325f4017155a3
Signed-off-by: Ryan Case <ryandcase@chromium.org>
Git-commit: 64a428077758383518c258641e81d57fcd454792
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


[msavaliy@codeaurora.org: Applied patch to downstream file - msm_geni_serial.c]
Signed-off-by: Mukesh Kumar Savaliya <msavaliy@codeaurora.org>