s390/pci: improve error handling during fmb (de)registration

Cleanup in zpci_fmb_enable_device when fmb registration fails. Also
don't free the fmb when deregistration fails in zpci_fmb_disable_device
but handle error situations when a function was hot-unplugged.

Also remove the mod_pci helper since it is no longer used.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Gerald Schaefer <gerald.schaefer@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>