net: ethernet: lpc_eth: Handle error for clk_enable

As the potential failure of the clk_enable(),
it should be better to check it and return error
if fails.

Fixes: b7370112 ("lpc32xx: Added ethernet driver")
Signed-off-by: Jiasheng Jiang <jiasheng@iscas.ac.cn>
Signed-off-by: David S. Miller <davem@davemloft.net>