PCI: switchtec: Add missing __iomem tag to fix sparse warnings

PCI: switchtec: Add missing __iomem tag to fix sparse warnings

Fix a missing __iomem tag in the init_pfn() function. This fixes a sparse
warning of the form:

  $ make C=2 drivers/pci/switch/
  drivers/pci/switch/switchtec.c:... incorrect type assignment(different address spaces)

Fixes: 080b47def5e5 ("MicroSemi Switchtec management interface driver")
Link: https://lore.kernel.org/r/20200728192434.18993-2-logang@deltatee.com
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>