mtd: nand: docg4: simplify error case

Other refactorings have left the 'fail' label much simpler, so it
shouldn't have to handle the failed allocation case.

This also fixes a -Wshadow warning.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Boris Brezillon <boris.brezillon@free-electrons.com>