net/mlx5e: Fix aRFS compilation dependency

en_arfs.o should be compiled only if both CONFIG_MLX5_CORE_EN
and CONFIG_RFS_ACCEL are enabled. en_arfs calls to rps_may_expire_flow
which is compiled only if CONFIG_RFS_ACCEL is defined.

Move en_arfs.o compilation dependency to be under CONFIG_MLX5_CORE_EN
and wrap the en_arfs.c content with ifdef of CONFIG_RFS_ACCEL.

Fixes: 1cabe6b0

 ('net/mlx5e: Create aRFS flow tables')
Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Reported-by: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>