Staging: vt6655: fix build when !CONFIG_WIRELESS_EXT

Fix this build error when CONFIG_WIRELESS_EXT is not set:
drivers/staging/vt6655/device_main.c:1153: error: 'struct net_device'
	has no member named 'wireless_handlers'

Cc: Forest Bond <forest@alittletooquiet.net>
Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>