drivers/mmc/core/: make 3 functions static

This patch makes the following needlessly global functions static:
- sd_ops.c: mmc_app_cmd()
- core.c: __mmc_release_bus()
- core.c: mmc_start_request()

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>