mtd: nand: fix trivial spelling error

Introduced by commit fde85cfd

 ("mtd: nand: Fix nand_command_lp() for
8bits opcodes") and I didn't have the heart to have Boris rewrite his
pull request just for that. Anyway, there's some value in having stable
commit hashes.

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