mmc: Fix mmc_send_status()

The mmc_send_status() function sets cmd.arg = 0. That's incorrect, so fix it.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Acked-by: Andy Fleming <afleming@freescale.com>
Tested-by: Lei Wen <adrian.wenl@gmail.com>