tty: n_tty: use MASK() for masking out size bits

In n_tty, there is already a macro to mask out top bits from ring buffer
counters. It is MASK() added some time ago. So use it more in the code
to make it more readable.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-7-jirislaby@kernel.org


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