cmd/ubi.c: Fix format warning

On 64bit platforms we would otherwise see:
../cmd/ubi.c: In function 'ubi_volume_read':
../cmd/ubi.c:359:16: warning: format '%u' expects argument of type 'unsigned int', but argument 2 has type 'size_t {aka long unsigned int}' [-Wformat=]

Fixes: 68c7025d99db ("cmd: ubi: print load size after establishing volume size")
Signed-off-by: Tom Rini <trini@konsulko.com>

cmd/ubi.c

@@ -356,7 +356,7 @@ int ubi_volume_read(char *volume, char *buf, size_t size)
 		size = vol->used_bytes;
 	}
 
-	printf("Read %u bytes from volume %s to %p\n", size, volume, buf);
+	printf("Read %zu bytes from volume %s to %p\n", size, volume, buf);
 
 	if (vol->corrupted)
 		printf("read from corrupted volume %d", vol->vol_id);