mmc: core: Fix unclocked access in get_xfer_remain/stop_request

The unclocked access could happen in the following scenario -

1. mmcqd thread is waiting for previously submitted io to be done
2. before this io is done, it is woken up due to is_urgent event
3. mmcqd thread is running and is processing the is_urgent event
4. Just about the same time the previously submitted io is done and
it sets is_done_rcv flag, tries to wake up mmcqd thread and release
host clock so as to gate the clocks.

If step 3 and 4 happen at the same time, then clocks will be gated off
before is_urgent is completely handled. Hence, make sure to hold the
clock before invoking get_xfer_remain and stop_request host->op.

Change-Id: If03177bf5e28cc217cc8e86b10d81d4adbab78b9
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>