mtd: rawnand: fix double free in atmel_pmecc_create_user()

The "user" pointer was converted from being allocated with kzalloc() to
being allocated by devm_kzalloc().  Calling kfree(user) will lead to a
double free.

Fixes: 6d734f1b ("mtd: rawnand: atmel: Fix possible memory leak")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>