wusb: fix error path for wusb_set_dev_addr()

Error path for wusb_set_dev_addr() was handled incorrectly. Fix it by
considering error only when return value is negative.

Signed-off-by: Anderson Lizardo <anderson.lizardo@indt.org.br>
Signed-off-by: David Vrabel <david.vrabel@csr.com>