firmware_loader: add a sanity check for firmware_request_builtin()

Right now firmware_request_builtin() is used internally only
and so we have control over the callers. But if we want to expose
that API more broadly we should ensure the firmware pointer
is valid.

This doesn't fix any known issue, it just prepares us to later
expose this API to other users.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Link: https://lore.kernel.org/r/20210917182226.3532898-4-mcgrof@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>