mmc: don't allow extra cmdline arguments

The "mmc rescan" command takes no arguments. However, executing
"mmc rescan 1" succeeds, leading the user to believe that MMC device 1
has been rescanned. In fact, the "current" MMC device has been
rescanned, and the current device may well not be 1. Add error-checking
to the "mmc" command to explicitly reject any extra command-line
arguments so that it's more obvious when U-Boot isn't doing what the
user thought they asked it to.

Signed-off-by: Stephen Warren <swarren@nvidia.com>