serial: mxs-auart: Index is unsigned

Fix the following warning when building with W=1 option:

drivers/tty/serial/mxs-auart.c: In function 'mxs_auart_tx_chars':
drivers/tty/serial/mxs-auart.c:272:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>