net: bridge: set error code on failure

Function br_sysfs_addbr() does not set error code when the call
kobject_create_and_add() returns a NULL pointer. It may be better to
return "-ENOMEM" when kobject_create_and_add() fails.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=188781

Signed-off-by: Pan Bian <bianpan2016@163.com>
Acked-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>