ath9k_htc: remove dead code in error path of ath9k_htc_txcompletion_cb

This clause is conditioned on htc_hdr != NULL, but it will only be NULL
when that check is reached.

Coverity: CID 114318

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>