ath9k: use the right hw on ath_tx_setup_buffer() for HT

When using virtual wiphys the base sc->hw was being used, the correct
hw is passed along the caller already so just use that.

Cc: Jouni.Malinen <Jouni.Malinen@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>