wifi: rtw88: simplify __rtw_tx_work()

Since 'ieee80211_txq_get_depth()' allows NULL for 2nd and
3rd arguments, simplify '__rtw_tx_work()' by dropping unused
'byte_cnt'. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20231102115606.69838-1-dmantipov@yandex.ru