sata_fsl: Update RX_WATER_MARK for TRANSCFG

RX_WATER_MARK sets the number of locations in Rx FIFO that can be used before
the transport layer instructs the link layer to transmit HOLDS. Note that it
can take some time for the HOLDs to get to the other end, and that in the
interim there must be enough room in the FIFO to absorb all data that could
arrive.

Update the new recommended value to 16.

Signed-off-by: Prabhakar Kushwaha <prabhakar@freescale.com>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>