tty: caif: do not use N_TTY_BUF_SIZE

tty: caif: do not use N_TTY_BUF_SIZE

N_TTY_BUF_SIZE -- as the name suggests -- is the N_TTY's buffer size.
There is no reason to couple that to caif's tty->receive_room. Use 4096
directly -- even though, it should be some sort of "SKB_MAX_ALLOC" or
alike. But definitely not N_TTY_BUF_SIZE.

N_TTY_BUF_SIZE is private and will be moved to n_tty.c later.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Link: https://lore.kernel.org/r/20250317070046.24386-4-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>