serial: msm_geni_serial: Fix softlock

Transfers were being divided into device FIFO sized (64 byte max)
operations which would poll for completion within a spin_lock_irqsave /
spin_unlock_irqrestore block. This both made things slow by waiting for
the FIFO to completely drain before adding further data and would also
result in softlocks on large transmissions.

This patch allows larger transfers with continuous FIFO additions as
space becomes available and removes polling from the interrupt handler.

(am from https://lore.kernel.org/patchwork/patch/1017608/

).

Change-Id: Icba9b7f7cc3fd8312e7f2473b3a5665ff8aedec2
Signed-off-by: Ryan Case <ryandcase@chromium.org>
Signed-off-by: Alok Chauhan <alokc@codeaurora.org>