to map, and technically this isn't allowed.
Functionally, it works OK (at least on x86) to call bus_dmamap_load with
a NULL data pointer and zero length, so this is primarily for correctness
and consistency with other drivers.
While here, remove check in isci_io_request_construct for nseg==0.
Previously, bus_dmamap_load would pass nseg==1, even for case where
buffer is NULL and length = 0, which allowed CAM_DIR_NONE CCBs
to get processed. This check is not correct though, and needed to be
removed both for the changes elsewhere in this patch, as well as jeff's
preliminary bus_dmamap_load_ccb patch (which uncovered all of this in
the first place).
MFC after: 3 days
dfbc6dc2fa