mmc: core: Fix NULL pointer dereference issue with mmc_blk_reset()

If the mmc_hw_reset() fails, then host->card might be NULL in some
cases. Hence, check for reset errors and report it to the caller so
that the current request can be aborted and also check for host->card
before accessing it so as to prevent NULL pointer dereference issue.

Change-Id: Iba0f0be314474e607a40383bc0b28eef66a31d63
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>