greybus: sdio: correct the usage of mmc request in work queues handler

The mmc request should assigned before use. Then
It should avoid freeing before using in mmc_request_done().

Signed-off-by: Phong Tran <tranmanphong@gmail.com>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Mark Greer <mgreer@animalcreek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>