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: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NEzequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: NArtem Bityutskiy <artem.bityutskiy@linux.intel.com>

drivers/mtd/ubi/block.c

@@ -201,7 +201,7 @@ static int ubiblock_read(struct ubiblock *dev, char *buffer,
 	int ret, leb, offset;
 	int bytes_left = len;
 	int to_read = len;
-	loff_t pos = sec << 9;
+	u64 pos = sec << 9;
 
 	/* Get LEB:offset address to read from */
 	offset = do_div(pos, dev->leb_size);