The CPSW uses DMA, so we should quiesce the device before leaving
barebox. This patch unregisters the CPSW properly on the device
remove callback. To do this we have to fix the error path in
cpsw_slave_setup, since this function can fail and we need a
known slave status in the remove function.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>