i40e: Refactor I40E_MDIO_CLAUSE* macros

The macros I40E_MDIO_CLAUSE22* and I40E_MDIO_CLAUSE45* are using I40E_MASK
together with the same values I40E_GLGEN_MSCA_STCODE_SHIFT and
I40E_GLGEN_MSCA_OPCODE_SHIFT to define masks.
Introduce I40E_GLGEN_MSCA_OPCODE_MASK and I40E_GLGEN_MSCA_STCODE_MASK
for both shifts in i40e_register.h and use them to refactor the macros
mentioned above.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Reviewed-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Reviewed-by: Aleksandr Loktionov <aleksandr.loktionov@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>