hwrng: bcm2835: fix MODULE_LICENSE tag

The MODULE_LICENSE macro invocation must use either "GPL" or "GPL v2",
but not "GPLv2" in order to be detected by the module loader.

This fixes the allmodconfig build error:

FATAL: modpost: GPL-incompatible module bcm2835-rng.ko uses GPL-only symbol 'platform_driver_unregister'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Dom Cobley <popcornmix@gmail.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Matt Mackall <mpm@selenic.com>
Cc: linux-rpi-kernel@lists.infradead.org
Cc: Herbert Xu <herbert@gondor.apana.org.au>