intel-ethernet: use correct module license

We recently updated all our SPDX identifiers to correctly
indicate our net/ethernet/intel/* drivers were always released
and intended to be released under GPL v2, but the MODULE_LICENSE
declaration was never updated.

Fix the MODULE_LICENSE to be GPL v2, for all our drivers.

Signed-off-by: Jesse Brandeburg <jesse.brandeburg@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>