mtd: nand: pxa3xx: Add ECC BCH correctable errors detection

This commit extends the ECC correctable error detection to include
ECC BCH errors. The number of BCH correctable errors can be any up to 16,
and the actual value is exposed in the NDSR register.

Therefore, we change some symbol names to refer to correctable or
uncorrectable (instead of single-bit or double-bit as it was in the
Hamming case) and while at it, cleanup the detection code slightly.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Daniel Mack <zonque@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>