staging: rtl8712: fix wrong function output

Return NULL from r8712_find_network() if no matched wlan_network
was found. Code with a bug:

while (plist != phead) {
	pnetwork = container_of(plist, struct wlan_network, list);
	plist = plist->next;
	if (!memcmp(addr, pnetwork->network.MacAddress, ETH_ALEN))
		break;
}
spin_unlock_irqrestore(&scanned_queue->lock, irqL);
return pnetwork;

In this code last processed pnetwork returned if list end was reached
and no pnetwork matched test condition.

Signed-off-by: Sergei Krainov <sergei.krainov.lkd@gmail.com>
Link: https://lore.kernel.org/r/20210409124611.GA3981@test-VirtualBox


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>