staging: mt7621-mmc: Cleanup includes

This commit cleans the includes in the sd.c file. Those which are not
needed are removed, the remaining ones are sorted alphabetically.

Signed-off-by: Christian Lütke-Stetzkamp <christian@lkamp.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>