mtd: docg3: fix use-after-free in docg3_release()

In the Linux kernel, the following vulnerability has been resolved:

mtd: docg3: fix use-after-free in docg3_release()

In docg3_release(), the docg3 pointer is obtained from

cascade->floors[0]->priv before the loop that calls

doc_release_device() on each floor. doc_release_device() frees the

docg3 struct via kfree(docg3) at line 1881. After the loop,

docg3->cascade->bch dereferences the already-freed pointer.

Fix this by accessing cascade->bch directly, which is equivalent

since docg3->cascade points back to the same cascade struct, and

is already available as a local variable. This also removes the

now-unused docg3 local variable.