serial: omap: enable RX and TX FIFO usage

enable RX FIFO for 16 characters and TX FIFO
for 16 spaces.

Tested-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>