drivers/mtd/nand/nand_spl_simple.c: Fix GCC 4.6 warnings

drivers/mtd/nand/nand_spl_simple.c: Fix GCC 4.6 warnings

Fix warnings for both cases:

definded CONFIG_SYS_NAND_HW_ECC_OOBFIRST:
nand_spl_simple.c: In function 'nand_read_page':
nand_spl_simple.c:156:6: warning: variable 'stat' set but not used
[-Wunused-but-set-variable]

not definded CONFIG_SYS_NAND_HW_ECC_OOBFIRST:
nand_spl_simple.c: In function 'nand_read_page':
nand_spl_simple.c:196:6: warning: variable 'stat' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Cc: Heiko Schocher <hs@denx.de>
Cc: Scott Wood <scottwood@freescale.com>
Acked-by: Heiko Schocher <hs@denx.de>
Acked-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>