mtd: physmap_of: error checking to prevent a NULL pointer dereference

This patch solves a NULL pointer dereference, this may occur if the tuple
is not mappable (jumps to continue in the for-loop). Out of the loop
possible results are:
- info->list_size == 0  if no of the tuples is mappable
- info->list_size == 1
- info->list_size > 1
If no one of the supplied tuples is mappable (info->list_size == 0) and
info->cmtd will not be set. But it is used in mtd_device_parse_register, OOPS!
actually it should generate an error in this case!

Signed-off-by: Anton Prins <anton.prins@nl.bosch.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>