GCC4,6: Squash warning in cmd_nand.c

cmd_nand.c: In function ‘do_nand’:
cmd_nand.c:490:7: warning: variable ‘chip’ set but not used
[-Wunused-but-set-variable]
cmd_nand.c:489:7: warning: variable ‘part’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Scott Wood <scottwood@freescale.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>