tty: switch receive_buf() counts to size_t

'size_t' is what receive_buf() expects and returns while handling count.
So switch to 'size_t'.

This renders both local 'count' and 'rcvd' in flush_to_ldisc() to be
size_t too.

Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230810091510.13006-12-jirislaby@kernel.org


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