hns: do not cast return value of napi_gro_receive to null

Basically no drivers care about the return value here, and there's no
__must_check that would make casting to void sensible, so remove it.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: David S. Miller <davem@davemloft.net>