switchtec: Fix minor bug with partition ID register

When a switch endpoint is configured without NTB, the mmio_ntb registers
will read all zeros.  However, in corner case configurations where the
partition ID is not zero and NTB is not enabled, the code will have the
wrong partition ID and this causes the driver to use the wrong set of
drivers.  To fix this we simply take the partition ID from the system info
region.

Reported-by: Dingbao Chen <dingbao.chen@microsemi.com>
Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>