net: gptp_md: Consider different resolutions regarding correction field

From https://www.ieee802.org/1/files/public/docs2007/as-garner-protocol-state-machines-frame-formats-0307.pdf

:
  The correctionField is the value of the correction
  measured in nanoseconds and multiplied by 2^16.

Zephyr uses nanoseconds for follow_up_correction_field.
Therefore, the value must be shifted by 2^16.

Signed-off-by: Martin Fletzer <martin.fletzer@kftec.at>