ethernet: sfc: Add Kconfig entry for vendor Solarflare

Since commit

  5a6681e2 ("sfc: separate out SFC4000 ("Falcon") support into new sfc-falcon driver")

there are two drivers for Solarflare devices, but both still show up
directly beneath "Ethernet driver support" in the Kconfig. Follow the
pattern of other vendors and group them beneath an own vendor Kconfig
entry for Solarflare.

Cc: Edward Cree <ecree@solarflare.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Edward Cree <ecree@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>