scsi: hisi_sas: Make max IPTT count equal for all hw revisions

There is a small optimisation to be had by making the max IPTT the same for
all hw revisions, that being we can drop the check for read and write
pointer being the same in the get free slot function.

Change v1 hw to have max IPTT of 4096 - same as v2 and v3 hw - and
drop hisi_sas_hw.max_command_entries.

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