After c11d8f89: "ath9k_htc: Simplify TX URB management" we no longer
assume that tx_buf is a non-null pointer.
Signed-off-by: NDan Carpenter <error27@gmail.com>
Acked-by: NSujith <Sujith.Manoharan@atheros.com>
Signed-off-by: NJohn W. Linville <linville@tuxdriver.com>