isci: cleanup/optimize pool implementation

The circ_buf macros are ~6% faster, as measured by perf, because they take
advantage of power-of-two math assumptions i.e. no test and branch for
rollover. Their semantics are clearer than the hidden side effects in pool.h
(like sci_pool_get() which hides an assignment).

Signed-off-by: Dan Williams <dan.j.williams@intel.com>

diff --cc drivers/scsi/isci/host.c
Simple merge

diff --cc drivers/scsi/isci/host.h
Simple merge

diff --cc drivers/scsi/isci/isci.h
Simple merge

diff --cc drivers/scsi/isci/pool.h
Simple merge