wifi: rtl8xxxu: extend wifi connected check to both interfaces

There are multiple places in the code where the current connection
status of wifi is checked. The driver will support two interfaces soon
and either one of them (or both) could be connected.

Convert all uses of (vif && vif->cfg.assoc) to a new helper
function rtl8xxxu_is_assoc() which checks both interfaces.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://msgid.link/20231222101442.626837-7-martin.kaistra@linutronix.de