serial: mxc: have putc use the TXFIFO

only waiting for TXEMPTY leads to corrupted messages going over the
wire - which is fixed by making use of the FIFO

this change is following the linux kernel uart driver
(drivers/tty/serial/imx.c), which also checks UTS_TXFULL
instead of UTS_TXEMPTY

Signed-off-by: Johannes Schneider <johannes.schneider@leica-geosystems.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>