mmc: return meaningful error codes from mmc_select_hwpart

Rather than just returning -1 everywhere, try to return something
meaningful from mmc_select_hwpart(). Note that most other MMC functions
don't do this, including functions called from mmc_select_hwpart(), so
I'm not sure how effective this will be. Still, it's one less place with
hard-coded -1.

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