mtd: fix warnings due to 64-bit partition support

commit 39ac34473f3c96e77cbe03a49141771ed1639486 ("cmd_mtdparts: use 64
bits for flash size, partition size & offset") introduced warnings
in a couple places due to printf formats or pointer casting.

This patch fixes the warnings pointed out here:
http://lists.denx.de/pipermail/u-boot/2013-October/164981.html

Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: York Sun <yorksun@freescale.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: Tom Rini <trini@ti.com>