greybus: sdio: some cleanups in command function

Some cleanups in gb_sdio_command function, ret does not need to be
initialize and mrq is already pointing to request, no need to get it
from host.

Signed-off-by: Rui Miguel Silva <rui.silva@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>