uwb: i1480-dfu: fix NULL-deref at probe

commit 4ce362711d78a4999011add3115b8f4b0bc25e8c upstream.

Make sure to check the number of endpoints to avoid dereferencing a
NULL-pointer should a malicious device lack endpoints.

Note that the dereference happens in the cmd and wait_init_done
callbacks which are called during probe.

Fixes: 1ba47da5 ("uwb: add the i1480 DFU driver")
Cc: Inaky Perez-Gonzalez <inaky.perez-gonzalez@intel.com>
Cc: David Vrabel <david.vrabel@csr.com>
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>