ata_std_qc_defer — Check whether a qc needs to be deferred
|struct ata_queued_cmd * qc|
Non-NCQ commands cannot run with any other command, NCQ or
not. As upper layer only knows the queue depth, we are
responsible for maintaining exclusion. This function checks
whether a new command
qc can be issued.
LOCKING: spin_lock_irqsave(host lock)