net: macb: unprepare clocks in case of failure

Unprepare clocks in case of any failure in fu540_c000_clk_init().

Fixes: c218ad55 ("macb: Add support for SiFive FU540-C000")
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>