cmd_mmc: make mmc dev always re-probe the HW

cmd_mmc: make mmc dev always re-probe the HW

Currently, U-Boot behaves as follows:

- Begin with no SD card inserted in "mmc 1"
- Execute: mmc dev 1
- This fails, since there is no card
- User plugs in an SD card
- Execute: mmc dev 1
- This still fails, since the HW isn't reprobed.

With this change, U-Boot behaves as follows:

- Begin with no SD card inserted in "mmc 1"
- Execute: mmc dev 1
- This fails, since there is no card
- User plugs in an SD card
- Execute: mmc dev 1
- The newly present SD card is detected

I know that "mmc rescan" will force the HW to be reprobed, but I feel it
makes more sense if "mmc dev" always reprobes the HW after selecting the
current MMC device. This allows scripts to just execute "mmc dev", and
not have to also execute "mmc rescan" to check for media presense.

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