[MTD] Fix build warnings (and debug build error) in nand_base.c

drivers/mtd/nand/nand_base.c: In function 'nand_transfer_oob':
drivers/mtd/nand/nand_base.c:909: warning: comparison of distinct pointer types lacks a cast
drivers/mtd/nand/nand_base.c: In function 'nand_do_read_oob':
drivers/mtd/nand/nand_base.c:1097: error: 'len' undeclared (first use in this function)
drivers/mtd/nand/nand_base.c:1097: error: (Each undeclared identifier is reported only once
drivers/mtd/nand/nand_base.c:1097: error: for each function it appears in.)
drivers/mtd/nand/nand_base.c: In function 'nand_fill_oob':
drivers/mtd/nand/nand_base.c:1411: warning: comparison of distinct pointer types lacks a cast

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>