at91_mci: Fix byte mode transitions.

The byte mode support fails to clear the byte mode bit in the command
register, possibly leaving byte mode enabled with the counters programmed
in non-byte mode.

Signed-off-by: Ville Syrjala <syrjala@sci.fi>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>