GCC4.6: Squash warning in nand_bbt.c

nand_bbt.c: In function ‘search_bbt’:
nand_bbt.c:465:6: warning: variable ‘bits’ set but not used
[-Wunused-but-set-variable]

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Scott Wood <scottwood@freescale.com>