staging/rts_pstor: remove braces {} in sd.c (reset_mmc_only)

fixed below checkpatch warnings.
-WARNING: braces {} are not necessary for single statement blocks

Signed-off-by: Toshiaki Yamane <yamanetoshi@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>