tg3: Restore likely() check in tg3_poll_msix()

When creating the new tg3_poll_msix() function, the likely() compiler
hint was dropped.  This patch reintroduces it.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Reviewed-by: Benjamin Li <benli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>