base-files: simplify setting device MAC

base-files: simplify setting device MAC

1. Move code above interface generation
   It results in more logical order. Device gets its config section
   above interface section.
2. Drop the loop
   We have separated code handling bridges now so $device should be
   guaranteed to contain a single device name.
3. Drop section name
   It's not required by netifd or LuCI & it's not needed by this script
   as $device contains a single device name now.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit e002179a6d2eea8640e262160030336bea54ec26)