wifi: rtl8xxxu: Fix use after rcu_read_unlock in rtl8xxxu_bss_info_changed

Commit a8b5aef2 ("wifi: rtl8xxxu: gen2: Enable 40 MHz channel width")
introduced a line where the pointer returned by ieee80211_find_sta() is
used after rcu_read_unlock().

Move rcu_read_unlock() a bit lower to fix this.

Fixes: a8b5aef2

 ("wifi: rtl8xxxu: gen2: Enable 40 MHz channel width")
Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/3c82ad09-7593-3be1-1d2c-e58505fb43cb@gmail.com