The reference says:

   If the ACK flag was set then send:
       <SN=received AN><CTL=RST>
   If the ACK flag was not set then send:
       <SN=0><AN=received SN+1 modulo 2><CTL=RST, ACK>
   (i.e. in both cases RST is needed)

The RST flag is set initially in the 'control' variable; so instead of
completely overwriting it, make sure the additional flags are OR-ed
to the current value.

This fix isn't related to any failed barebox<->bbremote interaction
(it really is a very rare corner case).

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>