mtd: nand: Fix regression in BBM detection

Commit c7b28e25

 ("mtd: nand: refactor BB 
marker detection") caused a regression in detection of factory-set bad 
block markers, especially for certain small-page NAND. This fix removes 
some unneeded constraints on using NAND_SMALL_BADBLOCK_POS, making the 
detection code more correct.

This regression can be seen, for example, in Hynix HY27US081G1M and
similar.

Signed-off-by: Brian Norris <norris@broadcom.com>
Tested-by: Michael Guntsche <mike@it-loops.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>