mtd: nand: orion: improve handling of optional clock

The clock gate used by orion_nand is not available on all platforms.
When getting this optional clock gate, the code masked all errors.
Let's be more precise here and actually only allow ENOENT.

EPROBE_DEFER is handled like any other error code since probe deferral
is not supported by drivers using module_platform_driver_probe().

Signed-off-by: Simon Baatz <gmbnomis@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>