GCC4.6: Squash warnings in onenand_base.c

Fix gcc 4.6 build warnings:
onenand_base.c: In function 'onenand_probe':
onenand_base.c:2577:6: warning: variable 'maf_id' set but not used
[-Wunused-but-set-variable]

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Lukasz Majewski <l.majewski@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>