net/macb: make clk_enable atomic

Use clk_prepare_enable/clk_disable_unprepare to be safe on SMP systems.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Fabio Estevam <festevam@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>