mv643xx_eth: Fix compile error for architectures without clk.

Commit 452503eb (ARM: Orion: Eth: Add clk/clkdev support.) broke
the building of the driver on architectures which don't have clk
support. In particular PPC32 Pegasos which uses this driver.

Add #ifdef around the clk API usage.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>