mmc: cqhci: verify the doorbell bit for DCMD before setting

Driver sets a tag bit in doorbell register to start transfer,
and if that bit is already set, that means tag is already have
request, and no new request should come to driver through that
tag again until on going request is completed.

So here, in code driver is reading the doorbell set bit to confirm tag
is free, if not then driver will BUG().
We make sure that the doorbell bit for any regular slot is not
set before ringing the doorbell for that slot, make sure of the
same for DCMD slot too.

Change-Id: I3d86c71ed1cd95f058148546cafc5304d1ac2f4b
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Ram Prakash Gupta <rampraka@codeaurora.org>