bridge: Fix incorrect variable assignment on error path in br_sysfs_addbr

This fixes the incorrect variable assignment on error path in
br_sysfs_addbr for when the call to kobject_create_and_add
fails to assign the value of -EINVAL to the returned variable of
err rather then incorrectly return zero making callers think this
function has succeededed due to the previous assignment being
assigned zero when assigning it the successful return value of
the call to sysfs_create_group which is zero.

Signed-off-by: Bastien Philbert <bastienphilbert@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>