target/cortex_a: fix waiting for target halted after step

target/cortex_a: fix waiting for target halted after step

Depending on adapter speed, the function cortex_a_poll() can take
time to execute and can complete successfully when the timeout is
already expired. Checking the timeout at function return causes a
timeout error while that could be avoided.

Check the target status at cortex_a_poll() return, before checking
for the timeout.

Change-Id: I4c1581f6e718298c566df7b1359255e16e3955d5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5111
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>