atmel_serial: RS485: receiving enabled when sending data

By default the atmel_serial driver in RS485 mode disables receiving data until
all data in the send buffer has been sent. This flag allows to receive data
even whilst sending data.

Signed-off-by: Bernhard Roth <redacted> 
Signed-off-by: Claudio Scordino <redacted>

Acked-by: Alan Cox <redacted>

(although when we have people having a serious meta discussion about four
 spaces, or a tab or neither rather than functionality I despair)

This is sensible functionality and relevant to various interfaces so
makes sense.