target/cortex_a: check dscr before timeout

target/cortex_a: check dscr before timeout

In function cortex_a_wait_dscr_bits() the last read on dscr gets
ignored in case of timeout, even if it finally provides the value
that would trigger a successful return.

Check the returned value before testing the timeout.
Also, print a message on failure reading dscr.

Change-Id: I261ac1545113db39374833a55be911a4da71d893
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5112
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>