staging: mt7621-mmc: Modify the return type of the function msdc_do_command

Modify the return type of the function msdc_do_command to static int
as the member error of the structure struct mmc_command is of type int.

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>