mmc: squash lines for immediate return

These functions can be much simpler by squashing lines for immediate
return.

For *_bind() callbacks, they will be a simple wrapper function of an
upper-level bind API.

For mmc_set_{boot_bus_width,part_conf}, they will be a wrapper of
mmc_switch().

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>