igb: this patch addes the sr-iov enablement option via num_vfs parameter

This code adds a module parameter called num_vfs which defines if the
driver should attempt to use sr-iov and if so how many VFs should be
enabled.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>