net: lan78xx: Switch to SPDX identifier

Adopt the SPDX license identifier headers to ease license compliance
management.

Change-Id: Ie766dbd4723e512bd525bd63cd03fc1cc9b589a5
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Git-commit: 6be665a56d366bc7577e6b8f9aaaba57e4c38c35
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git


Signed-off-by: Liangliang Lu <luliang@codeaurora.org>