nand_boot.c: Fix GCC 4.6 build warnings

Fix:
nand_boot.c: In function 'nand_read_page':
nand_boot.c:190:6: warning: variable 'stat' set but not used [-Wunused-but-set-variable]
nand_boot.c: In function 'nand_boot':
nand_boot.c:271:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]

Signed-off-by: Stefan Roese <sr@denx.de>