tty: n_tty: extract n_tty_wait_for_input()

n_tty_read() is a very long function doing too much of different stuff.
Extract the "wait for input" to a separate function:
n_tty_wait_for_input(). It returns an error (< 0), no input (0), or has
potential input (1).

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-11-jirislaby@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>