scsi: hisi_sas: Drop hisi_sas_hw.get_free_slot

In commit 1273d65f ("scsi: hisi_sas: change queue depth from 512 to
4096"), the depth of each queue is the same as the max IPTT in the system.

As such, as long as we have an IPTT allocated, we will have enough space on
any delivery queue.

All .get_free_slot functions were checking for space on the queue by
reading the DQ read pointer. Drop this, and also raise the code into common
code, as there is nothing hw specific remaining.

Signed-off-by: John Garry <john.garry@huawei.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>