mtd: nand: decode Hynix MLC, 6-byte ID length

Hynix has introduced a new ID decoding scheme for their newer MLC, some of
which don't support ONFI. The following devices all follow the pattern given in
the datasheet for Hynix H27UBG8T2B, p.22:

Hynix H27UAG8T2A
Hynix H27UBG8T2A
Hynix H27UBG8T2B

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>