ice(4): Enable WB_ON_ITR capability for VFs (#76)

Let VF driver know that HW supports WB_ON_ITR
feature. It allows polling mode driver such
as DPDK to configure an immediate write-back
of completed descriptors.

Signed-off-by: Krzysztof Galazka <krzysztof.galazka@intel.com>
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1573