cmd_mmc: Use init_mmc_device() from do_mmc_rescan()

The body of init_mmc_device() is now identical to that of do_mmc_rescan()
except for the error codes returned. Modify do_mmc_rescan() to simply
call init_mmc_device() and convert the error codes, to avoid code
duplication.

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