tty: serial: OMAP: ensure FIFO levels are set correctly in non-DMA mode

Ensure FIFO levels are set correctly in non-DMA mode (the default).
This patch will cause a receive FIFO threshold interrupt to be raised when
there is at least one byte in the RX FIFO.  It will also cause a transmit
FIFO threshold interrupt when there is only one byte remaining in the TX
FIFO.

These changes fix the receive interrupt problem and part of the
transmit interrupt problem.  A separate set of issues must be worked
around for the transmit path to have a basic level of functionality; a
subsequent patch will address these.

DMA operation is unaffected by this patch.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: Govindraj Raja <govindraj.r@ti.com>
Cc: Kevin Hilman <khilman@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>