mmc: sh_mmcif: Remove dummy PM resume callback

Unassigned system sleep callbacks were always treated the same as dummy
callbacks that just return zero.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>