mtd: nand: Fix various memory leaks in core

The nand_scan_ident() function is not expected to allocate resources,
and people are usually not calling nand_cleanup() if something fails
between nand_scan_ident() and nand_scan_tail().

Move all functions that may allocate resource to the nand_scan_tail()
path to prevent such resource leaks.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>