Earlier used to post the current command without checking queue full
     after backlog submissions. So, post the current command only after
     confirming the space in queue after backlog submissions.

     Maintain host write index instead of reading device registers
     to get the next free slot to post the command.

     Return -ENOSPC in queue full case.
Signed-off-by: Srikanth Jampala <Jampala.Srikanth@cavium.com>
Reviewed-by: Gadam Sreerama <sgadam@cavium.com>
Tested-by: Jha, Chandan <Chandan.Jha@cavium.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>