TTY: serial, use atomic_inc_return in ioc4_serial

We want to know the value of the atomic variable in intr_connect after
the increment. But atomic_inc doesn't, per definition, return the
value.  It is just a pure coincidence that ia64 defines atomic_inc as
atomic_inc_return.

So fix this mistake by using atomic_inc_return properly.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>