ice: fix PHY timestamp extraction for ETH56G

Fix incorrect PHY timestamp extraction for ETH56G.
It's better to use FIELD_PREP() than manual shift.

Fixes: 7cab44f1 ("ice: Introduce ETH56G PHY model for E825C products")
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Przemyslaw Korba <przemyslaw.korba@intel.com>
Tested-by: Pucha Himasekhar Reddy <himasekharx.reddy.pucha@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>