iwlegacy: fix bugs in change_interface

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.

Cc: stable@kernel.org [2.6.38+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>