ath9k_htc: dereferencing before check in hif_usb_tx_cb()

After c11d8f89: "ath9k_htc: Simplify TX URB management" we no longer
assume that tx_buf is a non-null pointer.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>