net: mvpp2: Fix a typo in the license

The proper string for this license is "GPL v2", instead of "GPLv2".
This commit fixes that.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>