PCI: switchtec: Simplify switchtec_dma_mrpc_isr()

The "ret" variable in switchtec_dma_mrpc_isr() is superfluous.  Remove it
and just return the value.  No functional change intended.

Link: https://lore.kernel.org/r/20221216162126.207863-2-helgaas@kernel.org


Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>