wifi: rtw89: Fix some error handling path in rtw89_core_sta_assoc()

'ret' is not updated after a function call in rtw89_core_sta_assoc().
This prevent error handling from working.

Add the missing assignment.

Fixes: e3ec7017 ("rtw89: add Realtek 802.11ax driver")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/7b1d82594635e4406d3438f33d8da29eaa056c5a.1668354547.git.christophe.jaillet@wanadoo.fr