ice: Process TSYN IRQ in a separate function

Simplify TSYN IRQ processing by moving it to a separate function and
having appropriate behavior per PHY model, instead of multiple
conditions not related to HW, but to specific timestamping modes.

When PTP is not enabled in the kernel, don't process timestamps and
return IRQ_HANDLED.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Karol Kolacinski <karol.kolacinski@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>