This matches licensing used by other BCM5301X files and is preferred as:
1) GPL 2.0+ makes it clearly compatible with Linux kernel
2) MIT is also permissive but preferred over ISC

Both files were fully developed by me. Commits touching them were signed
by Florian and Hauke due to submitting process only.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>