net: phy: mdio-mux-bcm-iproc: check clk_prepare_enable() return value

Check clk_prepare_enable() return value.

Fixes: 2c723044 ("net: phy: Add pm support to Broadcom iProc mdio mux driver")
Signed-off-by: Rayagonda Kokatanur <rayagonda.kokatanur@broadcom.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>