cxgb4: Disable SR-IOV support for PF4-7 for T5

All T5 adapters will only support VFs on PF0-3 despite the ability of the
hardware to support them on PF4-7.  This keeps our T4 and T5 adapters more
similar which simplifies host driver software.

Signed-off-by: Vipul Pandya <vipul@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>