mtd: mtdpart: use uint64_t instead of int for cur_off

The types of "offset" and "size" of "struct mtd_partition" are uint64_t,
while mtd_parse_partitions() uses int to work with these values. When
the offset reaches 2GB, it is interpreted as a negative value, which
leads to error messages like

mtd: partition "<partition name>" is out of reach -- disabled

eg. when using the "ubi part" command.

Fix this by using uint64_t for cur_off and cur_sz.

Signed-off-by: Martin Kaistra <martin.kaistra@linutronix.de>
Reviewed-by: Kurt Kanzenbach <kurt@linutronix.de>
Reviewed-by: Heiko Schocher <hs@denx.de>