tty: n_tty: unify counts to size_t

tty: n_tty: unify counts to size_t

Some count types are already 'size_t' for a long time. Some were
switched to 'size_t' recently. Unify the rest with those now.

This allows for some min_t()s to become min()s. And make one min()
an explicit min_t() as we are comparing signed 'room' to unsigned
'count'.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230827074147.2287-13-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>