mmc: Fix mmc_spi error on cmd->flags field

The recent removal of the cmd->flags field caused error in the
debuging code of mmc_spi. Fix this:

mmc_spi.c: In function 'mmc_spi_request':
mmc_spi.c:179:2: error: 'struct mmc_cmd' has no member named 'flags'

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Andy Fleming <afleming@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>