net: ethernet: sfc: Make subdir logic consistent with other vendors

Both SFC and SFC_FALCON depend on NET_VENDOR_SOLARFLARE, hence use the
latter to decide whether to descend into the sfc subdirectory.
Move the rule to descend into sfc/falcon to the sfc subdirectory.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Martin Habets <mhabets@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>