mtd: nand: rename '_raw' BBT scan functions

None of these scanning functions use MTD_OPS_RAW mode any more, so there's
really nothing 'raw' about them. Rename them to (hopefully) make the code
a little clearer.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>