iwlwifi: fix bugs in change_interface

commit a2b76b3b31568da9d281a393845f17689594ccdf upstream.

If change_interface gets invoked during a firmware
restart, it may crash; prevent that from happening
by checking if ctx->vif is assigned.

Additionally, in my initial commit I forgot to set
the vif->p2p variable correctly, so fix that too.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>