PCI/switchtec: Read all 64 bits of part_event_bitmap

The part_event_bitmap register is 64 bits wide, so read it with ioread64()
instead of the 32-bit ioread32().

Fixes: 52eabba5

 ("switchtec: Add IOCTLs to the Switchtec driver")
Link: https://lore.kernel.org/r/20190910195833.3891-1-logang@deltatee.com
Reported-by: Doug Meyer <dmeyer@gigaio.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: stable@vger.kernel.org	# v4.12+
Cc: Kelvin Cao <Kelvin.Cao@microchip.com>