mmc: queue: Remove BUG_ON() in cmdq_thread

If any discard requests fails, the mmc driver should not cause kernel
panic. The error handling and informing block layer about error will be
done by completion context.

Change-Id: I0c88ad851ad12c392a557bcb94d3b21dfc62d1f8
Signed-off-by: Vijay Viswanath <vviswana@codeaurora.org>