tty: move N_TTY_BUF_SIZE to n_tty

tty: move N_TTY_BUF_SIZE to n_tty

"N_TTY_BUF_SIZE" is private to n_tty and shall not be exposed to the
world. Definitely not in tty.h somewhere in the middle of "struct
tty_struct".

This is a remnant of moving "read_flags" to "struct n_tty_data" in
commit 3fe780b379fa ("TTY: move ldisc data from tty_struct: bitmaps").
But some cleanup was needed first (in previous patches).

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250317070046.24386-5-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>