linux/kernel: Remove MLX4_EN_VXLAN for 4.8

This option is no longer needed and has been removed in upstream commit
torvalds/linux@a831274a1346913c145797ddee6f39e30e061318.

Signed-off-by: aszlig <[e-mail]>

pkgs/os-specific/linux/kernel/common-config.nix

···
   LOGO n # not needed
   MEDIA_ATTACH y
   MEGARAID_NEWGEN y
-  ${optionalString (versionAtLeast version "3.15") ''
+  ${optionalString (versionAtLeast version "3.15" && versionOlder version "4.8") ''
     MLX4_EN_VXLAN y
   ''}
   MODVERSIONS y