sllin: If length is unknown read until timer occurs or break is received

sllin: If length is unknown read until timer occurs or break is received

If the length is unknown stop waiting for new data until the tty rx queue
is empty once is not working on fast systems where sllin_receive_buf is
called for each byte received. Instead wait up to 8 bytes, when the
timeout occurs or when a new break is received.

Signed-off-by: Alexander Stein <alexander.stein@systec-electronic.com>