vfio/pci: Make vfio_pci_regops->rw() return ssize_t

vfio/pci: Make vfio_pci_regops->rw() return ssize_t

The only implementation of this in IGD returns a -ERRNO which is
implicitly cast through a size_t and then casted again and returned as a
ssize_t in vfio_pci_rw().

Fix the vfio_pci_regops->rw() return type to be ssize_t so all is
consistent.

Fixes: a9c9230383fc ("vfio/pci: Add infrastructure for additional device specific regions")
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Max Gurtovoy <mgurtovoy@nvidia.com>
Link: https://lore.kernel.org/r/0-v3-5db12d1bf576+c910-vfio_rw_jgg@nvidia.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>