mmc: core: skip card initialization if power class selection fails

With current implementation of power class selection,
mmc_select_powerclass() should never fail. So treat any error
returned by this function as serious enough to skip the card
initialization.

Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>
Reviewed-by: Namjae Jeon <linkinjeon@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>