target: lakemon: probemode entry isn't instantaneous

target: lakemon: probemode entry isn't instantaneous

When testing with Intel Galileo GEN2 I have noticed a few iterations
where probemode entry took a little longer. At most I had to read
tapstatus twice. This patch uses a loop of up to 100 iterations to
wait for tapstatus to update with PM entry results.

Change-Id: I1c87d7dde715255b3fd2460d299b9493218533fc
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Reviewed-on: http://openocd.zylin.com/4013
Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>