isci: remove SCIC_DEBUG_ENABLED, and fixup an odd macro

This will be replaced by state machine tracepoints and should have been a part
of the logger removal.

Ran across scic_sds_port_decrement_request_count() which is an ugly macro
which silently hides accounting errors.  Turn it into a WARN_ONCE to see if it
ever triggers.

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