tty: fix routine name in ptmx_open()

At ptmx_open(), the 2nd parameter for check_tty_count() should
be "ptmx_open".

Signed-off-by: Hiroshi Shimamoto <h-shimamoto@ct.jp.nec.com>
Acked-by: Alan Cox <alan@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>