bus/pci: clear driver on detach

bus/pci: clear driver on detach

When probing, a bus is responsible for filling the driver field to
indicate the device is bound.
Let's clear this field when detaching to keep a consistent behavior.

This is not a fix per se, since the device is freed when detaching.
But at least clearing the field has been added to remind that the
driver field has a meaning for the EAL.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Thomas Monjalon <thomas@monjalon.net>