net: gptp: Fix sync timestamp callback registration
net: gptp: Fix sync timestamp callback registration

The logic was inverted which prevented the callback from ever being
registered.

Signed-off-by: Tomasz Gorochowik <265c7c25eca9aa9127910457b0aea7e793d329f1@antmicro.com>

zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,galak/zephyr,punitvara/zephyr,explora26/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,explora26/zephyr,ldts/zephyr,kraj/zephyr,nashif/zephyr,punitvara/zephyr,Vudentz/zephyr,galak/zephyr,ldts/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,finikorg/zephyr,ldts/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,kraj/zephyr,galak/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr