UBI: block: Use 'u64' for the 64-bit dividend

Fixes the following warning on ARCH=avr32:

  drivers/mtd/ubi/block.c: In function 'ubiblock_read':
  drivers/mtd/ubi/block.c:207: warning: comparison of distinct pointer types lacks a cast

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>