Check clk_prepare_enable() return value.

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