mtd: fix all checkpatch errors in nand_base.c

Artem: it is much easier to verify patches against nand_base.c with
       checkpatch.pl when nand_base.c itself does not have so many
       checkpatch.pl warnings.

Signed-off-by: Florian Fainelli <ffainelli@freebox.fr>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>