net: Fix GRO_HASH_BUCKETS assertion.

FIELD_SIZEOF() is in bytes, but we want bits.

Fixes: d9f37d01 ("net: convert gro_count to bitmask")
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>