ath5k: remove duplicated re-assignment to pointer 'tq'

Pointer tq is initialized with &ah->ah_txq[queue] and then a few
lines later is re-assigned the same value, hence this duplicate
assignment is redundant and can be removed.

Cleans up clang warning:
drivers/net/wireless/ath/ath5k/qcu.c:326:25: warning: Value stored
to 'tq' during its initialization is never read

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