luci-mod-network: don't implicitly move bridge opts into device sections

When setting bridge and device specific options such 'stp' or
'igmp_snooping', LuCI so far transparently created or reused a
`config device` section and set the corresponding option there.

In the case of bridges, this triggers multiple problems:

 - When implicitely creating a `config device` section referring to the
   bridge device, the legacy bridge configuration of the corresponding
   interface is disabled, causing a broken configuration on subsequent
   save operations

 - Netifd does not appear to properly merge bridge settings from config
   device and config interface sections, leading to an incoherent
   configuration state

In order to avoid that issue, do not automatically migrate bridge specific
options.

Fixes: #4948
Signed-off-by: Jo-Philipp Wich <jo@mein.io>