[NET]: Fix HH_DATA_OFF.

When the hardware header size is a multiple of HH_DATA_MOD, HH_DATA_OFF()
incorrectly returns HH_DATA_MOD (instead of 0). This affects ieee80211 layer
as 802.11 header is 32 bytes long.

Signed-off-by: Jiri Benc <jbenc@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>