net/atm: fix proc_mpc_write incorrect return value

[ Upstream commit d8bde3bf ]

Then the input contains '\0' or '\n', proc_mpc_write has read them,
so the return value needs +1.

Fixes: 1da177e4 ("Linux-2.6.12-rc2")
Signed-off-by: Xiaobo Liu <cppcoffee@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>