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>