ice: refactor ice_ptp_hw to make functions static

As following methods are not used outside ice_ptp_hw,
they can be made static:
ice_read_phy_reg_e822
ice_write_phy_reg_e822
ice_ptp_prep_port_adj_e822

Signed-off-by: Jan Sokolowski <jan.sokolowski@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>