mailbox: Prevent out-of-bounds access in fw_mbox_index_xlate()

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

mailbox: Prevent out-of-bounds access in fw_mbox_index_xlate()

Although it is guided that `#mbox-cells` must be at least 1, there are

many instances of `#mbox-cells = <0>;` in the device tree. If that is

the case and the corresponding mailbox controller does not provide

`fw_xlate` and of_xlate` function pointers, `fw_mbox_index_xlate()` will

be used by default and out-of-bounds accesses could occur due to lack of

bounds check in that function.