stlink: fix open AP for v2j37 and v3j7

stlink: fix open AP for v2j37 and v3j7

The new stlink firmware requires opening the AP before issuing
any operation.
In the current code we have a 'questionable' check about the core
model to set the TAR autoincrement, that is issued without opening
the AP, thus causing a STLINK_BAD_AP_ERROR.

Modify the AP open API to handle this case and open AP#0 before
the memory access to check the core model.

Change-Id: I576955b5094bd41d63ff1fbad7b4fd9433253321
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reported-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-on: http://openocd.zylin.com/5691
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>