pasemi_mac: mac_to_intf() error not noticed

mac_to_intf() can return -1 when no device or function is found, but when
mac->dma_if is unsigned. The error wasn't noticed.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: David S. Miller <davem@davemloft.net>