sfc: Revert SRIOV changes.

This reverts commits:

d92916f7 ("sfc: Own header for nic-specific sriov functions,")
25672dba ("sfc: Enable VF's via a write to the sysfs file
 sriov_numvfs")

As they break the build with SRIOV disabled and there is no
easy way to fix it the way things are arranged.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>