mtd: docg3: fix a possible memory leak of mtd->name

In case DOC_CHIPID_G3, mtd->name is not freed in err handling path,
which is alloced by kasprintf(). Fix this by using devm_kasprintf().

Fixes: ae9d4934

 ("mtd: docg3: add multiple floor support")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Boris Brezillon <bbrezillon@kernel.org>