tty: Fold one-line assign function into callers

Now that tty_ldisc_assign() is a one-line file-scoped function,
remove it and perform the simple assignment at its call sites.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>