cmd_mmc.c: check mmc_init() during mmc dev

mmc dev ${devnum} will return 0 as success even if there is no card
inserted. Booting script like tegra-common-post.h will call mmc dev
${devnum} to check the mmc device status, it always return 0 even if
mmc_init() fails.

Check mmc_init() return value let mmc dev command return failure
when mmc_init() fails.

Signed-off-by: Bryan Wu <pengw@nvidia.com>
[swarren: Always return 1 on error; hush doesn't appear to support
arbitrary return values. Fixed compilation error due to multiple
declaration of ret variable.]
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
---
v2:
* Always return 1 on error; hush doesn't appear to support
  arbitrary return values.
* Fixed compilation error due to multiple declaration of ret variable.