Using the MXS MCI driver with an eight bit capable eMMC results into the
'devinfo' message the interface uses '0' bits for data transfer:

 barebox:/ devinfo mxs_mci0
 resources:
 num   : 0
 start : 0x80034000
 size  : 0x00002000
 driver: mxs_mci
 bus: platform

  Interface
   Min. bus clock: 1476 Hz
   Max. bus clock: 48000000 Hz
   Current bus clock: 24000000 Hz
   Bus width: 0 bit

The eight bit interface width is stored internally as value '2'. And a two bit
'2' ends up into 0xfffffffe when used as an array index. Using an unsigned
field instead fixes this issue:

 barebox:/ devinfo mxs_mci0
 resources:
 num   : 0
 start : 0x80034000
 size  : 0x00002000
 driver: mxs_mci
 bus: platform

  Interface
   Min. bus clock: 1476 Hz
   Max. bus clock: 48000000 Hz
   Current bus clock: 24000000 Hz
   Bus width: 8 bit

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>