scsi: hisi_sas: Use devm_bitmap_zalloc() when applicable

'hisi_hba->slot_index_tags' is a bitmap. Use 'devm_bitmap_zalloc()' to
simplify code, improve the semantic, and avoid some open-coded arithmetic
in allocator arguments.

Link: https://lore.kernel.org/r/4afa3f71e66c941c660627c7f5b0223b51968ebb.1637961191.git.christophe.jaillet@wanadoo.fr


Acked-by: John Garry <john.garry@huawei.com>
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>