mmc: remove BUG() for MMC_BLK_NEW_REQUEST error handling.

Removed BUG() from mmc_blk_issue_rw_rq() because the execution flow never
reaches there and compiler warning resolved by adding 'default:' clause to
the switch(status).

Change-Id: I5a806a0b3b471085998457fdc8da8dc467f6452d
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>