ath6kl: fixup merge damage in ath6kl_mgmt_tx

  CC [M]  drivers/net/wireless/ath/ath6kl/cfg80211.o
drivers/net/wireless/ath/ath6kl/cfg80211.c:1838:2: warning: initialization from incompatible pointer type

Caused by commit e9f935e3...

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