liquidio: avoid doing useless work

Avoid doing useless work by making sure that the response_list is not empty
before scheduling work to process it.

Signed-off-by: Prasad Kanneganti <prasad.kanneganti@cavium.com>
Signed-off-by: Felix Manlunas <felix.manlunas@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>