mtd: onenand: remove unused variable assignments

These variable assignments are never used (the variables are either
never used or are overwritten before use). This resolves some compiler
warnings like the following:

    drivers/mtd/onenand/onenand_base.c: In function 'flexonenand_get_boundary':
    drivers/mtd/onenand/onenand_base.c:3532:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
    drivers/mtd/onenand/onenand_base.c: In function 'onenand_probe':
    drivers/mtd/onenand/onenand_base.c:3838:6: warning: variable 'maf_id' set but not used [-Wunused-but-set-variable]

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>