GCC47: Fix warning in cmd_nand.c

cmd_nand.c: In function ‘arg_off_size’:
cmd_nand.c:216:5: warning: ‘maxsize’ may be used uninitialized in this function [-Wmaybe-uninitialized]

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Cc: Wolfgang Denk <wd@denx.de>