PCI/switchtec: Add module parameter to request more interrupts

Seeing the we want to use more interrupts in the NTB MSI code
we need to be able allocate more (sometimes virtual) interrupts
in the switchtec driver. Therefore add a module parameter to
request to allocate additional interrupts.

This puts virtually no limit on the number of MSI interrupts available
to NTB clients.

Signed-off-by: Logan Gunthorpe <logang@deltatee.com>
Cc: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>