iwlagn: fix change_interface for P2P types

When an interface changes type to a P2P type,
iwlagn will erroneously set vif->type to the
P2P type and not the reduced/split type. Fix
this by keeping "newtype" in another variable
for the assignment to vif->type.

Cc: stable@kernel.org [2.6.38+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>