dfu: nand: Verify writes

Previously NAND writes were not verified and could fail silently.  Add
a verification step after all writes to NAND.

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Reviewed-by: Lukasz Majewski <l.majewski@samsung.com>
Tested-by: Heiko Schocher <hs@denx.de>
Acked-by: Heiko Schocher <hs@denx.de>