IB/mthca: Check alternate P_Key index when setting alternate path

Check that the alternate P_Key index is in range when setting the
alternate path for a QP.  Also make a cosmetic touch up to the debug
message printed when the main P_Key index is out of range.

Signed-off-by: Dotan Barak <dotanb@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>