[MTD] Fix a bad dependency in the Blackfin code

Fix a bad dependency in the Blackfin code on a RomFS config symbol that doesn't
exist upstream.

Signed-off-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Tested-by: Bernd Schmidt <bernd.schmidt@analog.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>