pci: allow multiple calls to pcim_enable_device()

There's no reason not to allow multiple calls to pcim_enable_device().
Calls after the first one can simply be noop.  All PCI resources will
be released when the initial pcim_enable_device() resource is
released.

This allows more flexibility to managed PCI users.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>