imxmmc: remove DEBUG definition

Removed DEBUG #define #undef, because module is automaticaly
compiled with -DDEBUG when CONFIG_MMC_DEBUG is defined.
Currently it just generates compiler warning about redefinition.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@teltonika.lt>
Acked-by: Pavel Pisa <pisa@cmp.felk.cvut.cz>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>