mmc: atmel-sdhci: fix the clk_enable call in case of no ops

If the clock driver does not offer a clk_enable ops, then the system will
return -ENOSYS.
The clk_enable works with CCF (common clock framework).
Some clocks in some cases (like the generic clock for some products: sama5d2)
do not have the clk_enable primitive, and in this case probing of the driver
will fail.
This patch changes the behavior to return an error in case there is really
an error, and not a missing primitive.
If the clock driver does not have an enable primitive, most likely clocks
are always enabled or enabled in the set_rate primitives.

Fixes: 81f16438 ("mmc: atmel-sdhci: enable the required generic clock")
Signed-off-by: Eugen Hristev <eugen.hristev@microchip.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>