IB/ipath: Prevent link-recovery code from negating admin disable

The link can be put in LINKDOWN_DISABLE state either locally or via a
MAD.  However, the link-recovery code will take it out of that state as
a side-effect of attempts to clear SerDes/XGXS issues.

We add a flag to indicate "link is down on purpose, leave it alone."

Signed-off-by: Michael Albaugh <michael.albaugh@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>