mtd: onenand_base: Fix a memory leak bug

In onenand_scan(), if CONFIG_MTD_ONENAND_VERIFY_WRITE is defined,
'this->verify_buf' is allocated through kzalloc(). However, it is not
deallocated in the following execution, if the allocation for
'this->oob_buf' fails, leading to a memory leak bug. To fix this issue,
free 'this->verify_buf' before returning the error.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>