net: macb: sort init_reset_optional() with other init()s

init_reset_optional() is somewhat oddly placed amidst the macb_config
struct definitions. Move it to a more reasonable location alongside
the fu540 init functions.

Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>