mtd: nand: fixup bounds checks for nand_{lock,unlock}()

Coverity noticed that these 'ret' assignments weren't being used. Let's
use them.

Note that nand_lock() and nand_unlock() are still not officially used by
any drivers.

Coverity CIDs #1227054 and #1227037

Signed-off-by: Brian Norris <computersforpeace@gmail.com>