nand_spl/nand_boot.c: Fix build warning

Fix:
nand_boot.c: In function 'nand_read_page':
nand_boot.c:150:6: warning: variable 'stat' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Anatolij Gustschin <agust@denx.de>
Signed-off-by: Scott Wood <scottwood@freescale.com>