UBI: block: Use ENOSYS as return value when CONFIG_UBIBLOCK=n

In order to have a way of distinguishing an invalid ioctl from a
not supported (but otherwise valid) ioctl, this commit changes the
return value of the ioctl stubs from ENOTTY to ENOSYS.

This will be useful to report more accurate error messages from
userspace tools.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>