mmc: mxs-mmc: Remove platform data

All MXS users have been converted to device tree and the board files have
been removed.

No need to keep platform data in the driver.

Also move bus_width declaration in the beggining of mxs_mmc_probe() to
avoid: 'warning: ISO C90 forbids mixed declarations and code'.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Marek Vasut <marex@denx.de>
Signed-off-by: Chris Ball <cjb@laptop.org>