mmc: Fix error handling path when there is an active Discard (DCMD)

In case of Discard requests, currently the error handler was
not waking up the cmdq-thread which is waiting for completion
even of discard requests in the issue context. This is happening
only when the error request is a non-discard request.
So this patch adds a way to wake up the issue thread waiting
on discard completion event.

Change-Id: Ic3b7ed55b0bb1707d3474986908bbe4431878f25
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>