tty: switch count in tty_ldisc_receive_buf() to size_t

tty: switch count in tty_ldisc_receive_buf() to size_t

It comes from both paste_selection() and tty_port_default_receive_buf()
as unsigned (int and size_t respectively). Switch to size_t to converge
to that eventually.

Return the count as size_t too (the two callers above expect that).

Switch paste_selection()'s type of 'count' too, so that the returned and
passed type match.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-13-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>