mmc: check the return value of mmc_select_mode_and_width()

Since commit 01298da3 ("mmc: Change mode when switching to a boot
partition"), errors in mmc_select_mode_and_width() are ignored.
The return value should be checked.

Fixes: 01298da3 ("mmc: Change mode when switching to a boot partition")
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>