ath9k_htc: Use anchors for REGOUT pipe

hif_usb_regout_cb() frees the given URB, which is
borked by design. Use an anchor to simplify URB
management.

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