CRIS: Fix RS485 delay handling.

Very recently, the RS485 interface has been fixed by adding two further
fields (see commit 1b633184).

Check the value of the flag SER_RS485_RTS_BEFORE_SEND before delaying.

Signed-off-by: Claudio Scordino <claudio@evidence.eu.com>
Signed-off-by: Jesper Nilsson <jesper.nilsson@axis.com>