drivers/stlink_usb: fix stlink_usb_read_regs() for API v2

drivers/stlink_usb: fix stlink_usb_read_regs() for API v2

API v2 implementation for command READALLREGS returns the status
in the first two bytes, followed by two bytes of padding.
This makes the reply 4 bytes longer and changes the offset of the
first register value to 4.

Fix it for the case API v2 and clean-up the management of the
return value.

Change-Id: I448c82bcc0baa72d66fdfe7f0c525b94f8a4468b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4824
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>