cmd_mmc: add force_init parameter to init_mmc_device()

This allows callers to inject mmc->has_init = 0 between finding the
MMC device, and calling mmc_init(), which forces mmc_init() to rescan
the HW. Future changes will use this feature.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>