cxgbe/iw_cxgbe: debug routines to dump STAG (steering tag) entries.

t4_dump_stag to dump hw state for a known STAG.

t4_dump_all_stag to dump hw state for all valid STAGs. This routine
walks the entire STAG region looking for valid entries and this can take
a while for some configurations.

MFC after: 1 week
Sponsored by: Chelsio Communications