PCI/switchtec: Move check event ID from mask_event() to switchtec_event_isr()

The event ID check doesn't depend on anything in the mask_all_events() to
mask_event() path.  Do it in switchtec_event_isr() to avoid the CSR read in
mask_event().

Link: https://lore.kernel.org/r/20200106190337.2428-6-logang@deltatee.com


Signed-off-by: Wesley Sheng <wesley.sheng@microchip.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>