bluetooth: mesh: brg_cfg: use IS_ENABLED macro optimally

If use `IS_ENABLED` macro inside if statements but not as preprocessor
directives, we can always compile the code for settings even if settings
are not used. This allows to reduce amount of firmwares we need to build
in CI to ensure that code compiles.

Signed-off-by: Pavel Vasilyev <pavel.vasilyev@nordicsemi.no>