mtd: nand: silence some shift wrap warnings

There are static checkers which complain when we declare variables as
64 bit bitfields but only use the lower 32 bits because of shift
wrapping.  In this case "len" is declared as u64 as opposed to unsigned
long or something which might be 32 bits.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>