spi/pl022: fix stop queue procedure

This fix prevents queue being marked as "stopped", if data exists
in the queue list.

Signed-off-by: Grzegorz Sygieda <grzegorz.sygieda@tieto.com>
Signed-off-by: Lukasz Baj <lukasz.baj@tieto.com>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>