net: ethernet: 8390: Replace GPL 2.0 boilerplate with SPDX identifier

The boilerplate refers to COPYING in the top-level directory of kernel
tree. Replace it with corresponding SPDX license identifier.

Cc: Donald Becker <becker@scyld.com>
Cc: Peter De Schrijver <p2@mind.be>
Cc: Topi Kanerva <topi@susanna.oulu.fi>
Cc: Alain Malek <Alain.Malek@cryogen.com>
Cc: Bruce Abbott <bhabbott@inhb.co.nz>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Richard Fontana <rfontana@redhat.com>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>