virtio: fix build with mempool debug enabled

virtio: fix build with mempool debug enabled

The mempool header forces error on -Wcast-qual:
	error: cast discards ‘const’ qualifier from pointer target type

Let's fix it by removing const qualifier of pci driver from commit
	5e9f6d1340ff ("pci: reference driver structure for each device")
It's needed because the driver flags are changed depending on using uio or not.
Actually these driver flags should be directly attached to each device.

Fixes: da978dfdc43b ("virtio: use port IO to get PCI resource")

Signed-off-by: Thomas Monjalon <thomas.monjalon@6wind.com>
Acked-by: Changchun Ouyang <changchun.ouyang@intel.com>