ethtool: Add n-tuple string length to drvinfo and return it

The drvinfo struct should include the number of strings that
get_rx_ntuple will return.  It will be variable if an underlying
driver implements its own get_rx_ntuple routine, so userspace
needs to know how much data is coming.

Signed-off-by: Peter P Waskiewicz Jr <peter.p.waskiewicz.jr@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>