at76c50x-usb: fix warning caused by at76_mac80211_tx now returning void

  CC [M]  drivers/net/wireless/at76c50x-usb.o
drivers/net/wireless/at76c50x-usb.c: In function ‘at76_mac80211_tx’:
drivers/net/wireless/at76c50x-usb.c:1759:4: warning: ‘return’ with a value, in function returning void

This is fallout from commit 7bb45683
("mac80211: make tx() operation return void").

Signed-off-by: John W. Linville <linville@tuxdriver.com>