net: phy: smsc: fix clk error handling

Commit bedd8d78 ("net: phy: smsc: LAN8710/20: add phy refclk in
support") added the phy clk support. The commit already checks if
clk_get_optional() throw an error but instead of returning the error it
ignores it.

Fixes: bedd8d78

 ("net: phy: smsc: LAN8710/20: add phy refclk in support")
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20210111085932.28680-1-m.felsch@pengutronix.de
Signed-off-by: Jakub Kicinski <kuba@kernel.org>