net: phy: allow bcm84881 to be a module

Now that the phylib module loading issue has been resolved, we can
allow this PHY driver to be built as a module.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>