sfc: Siena: Disable write-combining when SR-IOV is enabled

If SR-IOV is enabled by firmware, even if it is not enabled in the PCI
capability, TX pushes using write-combining may be corrupted.

We want to know whether it is enabled before mapping the NIC
registers, and even if PCI extended capabilities are not accessible.
Therefore, we look for the MSI capability, which is removed if SR-IOV
is enabled.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>