net: macb: change return type for gem_ptp_set_one_step_sync()

gem_ptp_set_one_step_sync() always returns zero thus change its return
type to void.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220608080818.1495044-1-claudiu.beznea@microchip.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>