mtdchar: Register the full range of minor numbers

register_chrdev() registers minor numbers up to 255, but we can now
potentially have much larger numbers.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>