bridge: return boolean instead of integer in br_multicast_is_router

Return statements in functions returning bool should use
true/false instead of 1/0.

This issue was detected with the help of Coccinelle.

Fixes: 85b35269

 ("bridge: Fix build error when IGMP_SNOOPING is not enabled")
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Reviewed-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>