net: bridge: fix sign bug

ipv6_skip_exthdr() can return error code that is below zero.
'offset' is unsigned, so it makes no sense.
ipv6_skip_exthdr() returns 'int' so we can painlessly change type of
offset to int.

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>