net: lpc_eth: Check clk_prepare_enable() error

clk_prepare_enable() may fail, so we should better check its return
value and propagate it in the case of failure

While at it, replace __lpc_eth_clock_enable() with a plain
clk_prepare_enable/clk_disable_unprepare() call in order to
simplify the code.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Vladimir Zapolskiy <vz@mleia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>