net: sched: avoid newline at end of message in NL_SET_ERR_MSG_MOD

Fix following coccicheck warning:
./net/sched/act_api.c:277:7-49: WARNING avoid newline at end of message
in NL_SET_ERR_MSG_MOD

Signed-off-by: Wan Jiabing <wanjiabing@vivo.com>
Signed-off-by: David S. Miller <davem@davemloft.net>