mmc: block: cmdq discard should use softirq completion path

Currently cmdq discard requests are not using same
softirq completion path as other cmdq request. So there
is no way to detect any error and handle it in case
of error of DCMD requests.

Make cmdq discard requests use blk_complete_requests
which complete via softirq completion path.

Change-Id: I1e03c81bc6fee8266cf1c86bada015e548770d7a
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>
[xiaonian@codeaurora.org: fixed trivial merge conflicts &
one compilation error, changed REQ_FLUSH to REQ_PREFLUSH
as new defined in msm-4.9]
Signed-off-by: Xiaonian Wang <xiaonian@codeaurora.org>