mmc: Makefile: Fix EXTRA_CFLAGS assignment

The EXTRA_CFLAGS assignment in mmc/Makefile was not accomplishing
anything because this flag only has effect on sources at the same level
as the makefile (i.e., per directory). Since card/, core/, and host/
rely on MMC_DEBUG, the subdir-ccflags-y variant seems to be the
appropriate choice.

Signed-off-by: matt mooney <mfm@muteddisk.com>
Signed-off-by: Chris Ball <cjb@laptop.org>