uwb: hwa-rc: fix NULL-deref at probe

commit daf229b1 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 start callback which is called
during probe.

Fixes: de520b8b ("uwb: add HWA radio controller 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: Greg Kroah-Hartman <gregkh@linuxfoundation.org>