mtd: fix false positive "Offset exceeds device limit" error

Since commit 09c32807 (mtd, nand: Move common functions from
cmd_nand.c to common place), NAND commands would not work at all
on large devices.

    => nand read 80000000 10000 10000

    NAND read: Offset exceeds device limit
    => nand erase 100000 100000

    NAND erase: Offset exceeds device limit

The type of the "size" of "struct mtd_info" is uint64_t, while
mtd_arg_off_size() and mtd_arg_off() treat chipsize as int type.
The chipsize is wrapped around if the argument is given with 2GB
or larger.

Acked-by: Heiko Schocher <hs@denx.de>
Acked-by: Scott Wood <scottwood@freescale.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>