net: bridge: convert mtu_set_by_user to a bit

Convert the last remaining bool option to a bit thus reducing the overall
net_bridge size further by 8 bytes.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>