bridge: correct the comment for file br_sysfs_br.c

br_sysfs_if.c is for sysfs attributes of bridge ports, while br_sysfs_br.c
is for sysfs attributes of bridge itself. Correct the comment here.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>