net: ethernet: i825xx: sun3_8256: Add SPDX license identifier

The boilerplate reads that sun3_8256 driver is an extension to Linux
kernel core, hence add SPDX license identifier for GPL 2.0.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michael Hipp <hippm@informatik.uni-tuebingen.de>
Cc: Sam Creasey <sammy@sammy.net>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>