net/virtio: do not limit packed ring size

Virtio spec only set rule that packed ring maximum size is up to 2^15
entries. Should not limit packed ring size to power of two.

Fixes: aea29aa5d37b ("net/virtio: enable packed virtqueues by default")
Cc: stable@dpdk.org
Signed-off-by: Marvin Liu <yong.liu@intel.com>
Reviewed-by: Tiwei Bie <tiwei.bie@intel.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>