net: bridge: Slightly optimize 'find_portno()'

The 'inuse' bitmap is local to this function. So we can use the
non-atomic '__set_bit()' to save a few cycles.

While at it, also remove some useless {}.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Nikolay Aleksandrov <nikolay@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>