mac80211: Deinline drv_switch_vif_chanctx()

With this .config: http://busybox.net/~vda/kernel_config_ALLYES_Os

,
after deinlining the function size is 821 bytes and there are
2 callsites, reducing code size by about 800 bytes.

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: Johannes Berg <johannes.berg@intel.com>
CC: John Linville <linville@tuxdriver.com>
CC: Michal Kazior <michal.kazior@tieto.com>
CC: linux-wireless@vger.kernel.org
CC: linux-kernel@vger.kernel.org
[adjust code-style a bit]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>