ath6kl: remove redundant null pointer check on send_pkt

The check for send_pkt being NULL is redundant before the call
to htc_reclaim_txctrl_buf, therefore it should be removed. This was
detected by static analysis by cppcheck.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>