ethtool: add ethtool_intersect_link_masks

This function provides a way to intersect two link masks together to
find the common ground between them.  For example in i40e, the driver
first generates link masks for what is supported by the PHY type.  The
driver then gets the link masks for what the NVM supports.  The
resulting intersection between them yields what can truly be supported.

Signed-off-by: Alan Brady <alan.brady@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>