igb: Add MII write support

To facilitate running PHY parametric tests, add support for the SIOCSMIIREG
ioctl. This allows a userspace application to write to the PHY registers
to enable the test modes.

Signed-off-by: Jackie Jone <jackie.jone@alliedtelesis.co.nz>
Acked-by: Jacob Keller <jacob.e.keller@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>
Link: https://lore.kernel.org/r/20240618213330.982046-1-anthony.l.nguyen@intel.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>