serial: mxs-auart: set the FIFO size to DMA buffer size

When DMA is enabled (with hardware flow control enabled) the FIFO size
must be set to the size of the DMA buffer, as this is the size the tty
subsystem can use.

Signed-off-by: Hector Palacios <hector.palacios@digi.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>