tty/serial: atmel: split tx and rx paths

Split TX and RX paths to not schedule RX tasklet on TX events and vice versa.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>