PCI: cadence: Fix find_first_zero_bit() limit

The ep->ob_region_map bitmap is a long and it has BITS_PER_LONG bits.

Link: https://lore.kernel.org/r/20220315065829.GA13572@kili


Fixes: 37dddf14 ("PCI: cadence: Add EndPoint Controller driver for Cadence PCIe controller")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>