bcm63xx_enet: properly prepare/unprepare clocks before/after usage

Use clk_prepare_enable/disable_unprepare calls in preparation for
switching to the generic clock framework.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Acked-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>