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: Ic0fa73f189f3f27df24f36841e85265e739c3761
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>