net/phy/bcm87xx: Add MODULE_LICENSE("GPL") to GPL driver

Currently the driver has no MODULE_LICENSE attribute in its source which
results in a kernel taint if I load this:

root@(none):~# modprobe bcm87xx
bcm87xx: module license 'unspecified' taints kernel.

Since the first lines of the source code clearly state:
 * This file is subject to the terms and conditions of the GNU General
 * Public License.  See the file "COPYING" in the main directory of this
 * archive for more details.
I think it's safe to add the MODULE_LICENSE("GPL") macro and thus remove
the kernel taint.

Cc: stable@vger.kernel.org
Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: David S. Miller <davem@davemloft.net>