net: rtl8139: Fill in SPDX tag

net: rtl8139: Fill in SPDX tag

The rtl8139 driver is derived from Etherboot driver, which is in
turn derived from Linux 8139too driver added in Linux 2.2.18pre14.
An undocumented change in Linux 2.4.10.2 added a new field to the
driver, MODULE_LICENSE("GPL"); . According to current Linux kernel
licensing rules, Documentation/process/license-rules.rst, quote:

	"GPL"	Module is licensed under GPL version 2. This
		does not express any distinction between
		GPL-2.0-only or GPL-2.0-or-later. The exact
		license information can only be determined
		via the license information in the
		corresponding source files.

And since the code does not contain any "future" clause, the tag
is therefore GPL-2.0 only.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>