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) <[e-mail]>
Acked-by: Jakub Kicinski <[e-mail]>
Cc: Andrew Lunn <[e-mail]>
Cc: David S. Miller <[e-mail]>
Cc: Eric Dumazet <[e-mail]>
Cc: Paolo Abeni <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/net/caif/caif_serial.c

···
 	ser->tty = tty_kref_get(tty);
 	ser->dev = dev;
 	debugfs_init(ser, tty);
-	tty->receive_room = N_TTY_BUF_SIZE;
+	tty->receive_room = 4096;
 	tty->disc_data = ser;
 	set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
 	rtnl_lock();