s390/pci: try harder to modify a function

In rare situations a PCI function can report a busy condition
when we issue the modify pci function command. A temporary busy
condition can exceed 1 second but not 2 seconds. Increase the
time until we report an error to 2 seconds. Also increase the
time we sleep between the retries to reduce the load in this
case.

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