wifi: rtlwifi: simplify rtl_action_proc() and rtl_tx_agg_start()

Since 'drv_priv' is an in-place member allocated at the end of
'struct ieee80211_sta', it can't be NULL and so relevant checks
in 'rtl_action_proc()' and 'rtl_tx_agg_start()' may be dropped.
Compile tested only.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

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/20231113144734.197359-2-dmantipov@yandex.ru