nandwrite: clean up length types

nandwrite: clean up length types

We use 'int' in many places to represent offsets/sizes.  That obviously
does not play well with larger NAND devices on 32bit systems.  Instead,
use the right type as needed:
 - long long to represent the length of the image
 - use fstat() rather than lseek();lseek(); to get the length of the image
 - use size_t/ssize_t when working with read()
 - tweak the printf formats as needed

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>