s390/pci: adjust IOMAP_MAX_ENTRIES

ZPCI_IOMAP_MAX_ENTRIES is off by one. Let's adjust this
for the sake of correctness.

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>