rtw89: Fix variable dereferenced before check 'sta'

The pointer rtwsta is dereferencing pointer sta before sta is being null
checked. Fix this by assigning sta->drv_priv to rtwsta only if sta is not
NULL, otherwise just NULL.

Fixes: e3ec7017 ("rtw89: add Realtek 802.11ax driver")
Reported-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20211022061242.8383-1-pkshih@realtek.com