drivers/mtd/nand/nand_spl_load.c: Fix GCC 4.6 warning

Fix:
nand_spl_load.c: In function 'nand_boot':
nand_spl_load.c:31:6: warning: variable 'ret' 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>