Staging: wlan-ng: fix p80211wext_mhz_to_channel for channel 14

Channel 14 is 2484 Mhz (cf p80211wext_channel_to_mhz) so this patch
corrects what seems to be just a typo.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>