arm: socfpga: scan: Clean up scan_chain_engine_is_idle()

Rework this function so it's clear that it is only polling for certain
bits to be cleared. Add kerneldoc. Fix it's return value to be either
0 on success and -ETIMEDOUT on error and propagate this through the
scan manager code.

Signed-off-by: Marek Vasut <marex@denx.de>
Acked-by: Dinh Nguyen <dinguyen@opensource.altera.com>