sfc: support VI strides other than 8k

Medford2 can also have 16k or 64k VI stride.  This is reported by MCDI in
 GET_CAPABILITIES, which fortunately is called before the driver does
 anything sensitive to the VI stride (such as accessing or even allocating
 VIs past the zeroth).

Signed-off-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>