xen-pciback: First reset, then free.

We were doing the operations of freeing and reset in the wrong
order. Granted nothing broke because the reset functions just
set bar->which = 0.

But nonethless this was incorrect.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>