net/at91_ether: prepare and unprepare clock

The clock is enabled without being prepared, this leads to:

WARNING: CPU: 0 PID: 1 at drivers/clk/clk.c:889 __clk_enable+0x24/0xa8()

and a non working ethernet interface.

Use clk_prepare_enable() and clk_disable_unprepare() to handle the clock.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>