IB/usnic: Support more QP state transitions

They were already implemented at a lower layer, but the upper level
routine placed arbitrary restrictions on which transitions were
permitted.  Simplify the state machine logic to live wholly in
usnic_ib_qp_grp_modify.

Signed-off-by: Dave Goodell <dgoodell@cisco.com>
Reviewed-by: Reese Faucette <rfaucett@cisco.com>
Reviewed-by: Xuyang Wang <xuywang@cisco.com>
Signed-off-by: Nelson Escobar <neescoba@cisco.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>