mmc: core: Fix comparison issue in mmc_compare_ext_csds

Found this issue during code review. Actually, there are two issues which
both compensate together in lucky case.  In unlucky case the bus width
probing might not work as expected.

Signed-off-by: Jurgen Heeks <jurgen.heeks@nokia.com>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>