tty: serial: msm_geni_serial: Requeue buffer after handling the data

Sometimes the serial engine may generate an interrupt after all the data
is handled and when the engine is idle for a configured time. Currently
the driver does not requeue the buffer back into the serial engine. This
leads to stall in the RX path.

Requeue the buffer as long as it is valid and all the data in the buffer
are handled.

CRs-Fixed: 2093963
Change-Id: Idd9698c94d71f4742ea0b8b0b0f09ae6ff8e9930
Signed-off-by: Karthikeyan Ramasubramanian <kramasub@codeaurora.org>