tty: n_tty: simplify chars_in_buffer()

The 'if' in chars_in_buffer() is misleadingly inverted. And since the
only difference is the head used for computation, cache the head using
ternary operator. And use that in return directly.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-11-jirislaby@kernel.org


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