Fix logic error in rfkill_check_duplicity

> I'll have a prod at why the [hso] rfkill stuff isn't working next

Ok, I believe this is due to the addition of rfkill_check_duplicity in
rfkill and the fact that test_bit actually returns a negative value
rather than the postive one expected (which is of course equally true).
So when the second WLAN device (the hso device, with the EEE PC WLAN
being the first) comes along rfkill_check_duplicity returns a negative
value and so rfkill_register returns an error. Patch below fixes this
for me.

Signed-Off-By: Jonathan McDowell <noodles@earth.li>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: John W. Linville <linville@tuxdriver.com>