iwlwifi: do not nulify ctx->vif on reset

iwlwifi: do not nulify ctx->vif on reset

commit 8db4c7e25d153fb049e81715d72fa3be3a0c3b69 upstream.

ctx->vif is dereferenced in different part of iwlwifi code, so do not
nullify it.

This should address at least one of the possible reasons of WARNING at
iwlagn_mac_remove_interface, and perhaps some random crashes when
firmware reset is performed.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Backported to 3.2:
 - Change filename iwl-mac80211.c to iwl-core.c
 - Change context in iwlagn_prepare_restart()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>