net: fec: Do proper error checking for optional clks

An error code returned by devm_clk_get() might have other meanings than
"This clock doesn't exist". So use devm_clk_get_optional() and handle
all remaining errors as fatal.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>