cmd_mmc: default to HW partition 0 if not specified

cmd_mmc: default to HW partition 0 if not specified

Currently, "mmc dev 0" does not change the selected HW partition. I
think it makes more sense if "mmc dev 0" is an alias for "mmc dev 0 0",
i.e. that HW partition 0 (main data area) is always selected by default
if the user didn't request a specific partition. Otherwise, the following
happens, which feels wrong:

Select HW partition 1 (boot0):
mmc dev 0 1

Doesn't change the HW partition, so it's still 1 (boot0):
mmc dev 0

With this patch, the second command above re-selects the main data area.

Note that some MMC devices (i.e. SD cards) don't support HW partitions.
However, this patch still works, since mmc_start_init() sets the current
partition number to 0, and mmc_select_hwpart() succeeds if the requested
partition is already selected.

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