ata_scmd_need_defer — Check whether we need to defer scmd
ATA device to which the command is addressed
Is the command IO (and thus possibly NCQ)?
NCQ and non-NCQ commands cannot run together. As upper layer only knows the queue depth, we are responsible for maintaining exclusion. This function checks whether a new command can be issued to dev.
spin_lock_irqsave(host lock)
1 if deferring is needed, 0 otherwise.