adi_v5: enforce check on AP number value

adi_v5: enforce check on AP number value

The AP number value is restricted in 8 bits unsigned by ADI-v5
specification. Nevertheless, an "invalid" value is used by
target cortex-m to force an automatic detection of the AP.

Replace magic numbers by using new macros for AP max number and
for the value of AP invalid.
Check the value passed through -ap-num flag during configuration.

Change-Id: Ic19a367db0ab11c0ebd070750eca0647d25279a5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4668
Tested-by: jenkins
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>