uwb: use uwb_rsv_callback instead of calling rsv->callback directly

Use uwb_rsv_callback wrapper instead of calling rsv->callback directly.
uwb_rsv_callback checks for NULL and is used by other callers of the
callback routine.

Signed-off-by: Thomas Pugliese <thomas.pugliese@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>