cfg80211: fix compile error with CONFIG_CFG80211_DEBUGFS

If CONFIG_CFG80211_DEBUGFS is enabled and CONFIG_MAC80211_DEBUGFS is
not, compilation fails in net/wireless/debugfs.c:

net/wireless/debugfs.c: In function 'cfg80211_debugfs_drv_add':
net/wireless/debugfs.c:117: error: 'struct cfg80211_registered_device'
has no member named 'debugfs'

The debugfs filed is needed if and only if CONFIG_CFG80211_DEBUGFS is
enabled, so use that instead of CONFIG_MAC80211_DEBUGFS.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>