netdevice.h: check for CONFIG_WLAN instead of CONFIG_WLAN_80211

In "wireless: remove WLAN_80211 and WLAN_PRE80211 from Kconfig" I
inadvertantly missed a line in include/linux/netdevice.h.  I thereby
effectively reverted "net: Set LL_MAX_HEADER properly for wireless." by
accident. :-(  Now we should check there for CONFIG_WLAN instead.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Reported-by: Christoph Egger <siccegge@stud.informatik.uni-erlangen.de>
Cc: stable@kernel.org