m68k: coldfire: return success for clk_enable(NULL)

The clk_enable is supposed work when CONFIG_HAVE_CLK is false, but it
returns -EINVAL.  That means some drivers fail during probe.

[    1.680000] flexcan: probe of flexcan.0 failed with error -22

Fixes: c1fb1bf6 ("m68k: let clk_enable() return immediately if clk is NULL")
Fixes: bea8bcb1 ("m68knommu: Add support for the Coldfire m5441x.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>