ata_std_qc_defer — Check whether a qc needs to be deferred
ATA command in question
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.
spin_lock_irqsave(host lock)
ATA_DEFER_* if deferring is needed, 0 otherwise.