luci-mod-admin-full: properly reset internal device form field

In some cases the hidden internal device field was not reset, e.g. after
aborting a wifi scan and using the browser back buttons to navigate to the
overview page again.

In such a case, the previous device hidden field was still present and a new
one getting created, causing further wireless scan attempts to get invoked
with multiple radio names as parameter which fails.

Fix this issue by using the new generic cbi_submit() helper any by dropping
the faulty wifi_action() function.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>