wifi: cancel autoscan timeout when removing device

Usually autoscan is cancelled when the device is disabled, but when
wpa_supplicant leaves D-Bus wifi_disable() isn't called and thus
autoscan timeout isn't cleared. The timeout callback may access
released memory when it's activated.

Add stop_autoscan() call to wifi_remove() to make sure there are no
pending timeouts when the autoscan struct is released.