CMSIS-DAP: print a debug message when the USB product string cannot be read

CMSIS-DAP: print a debug message when the USB product string cannot be read

As suggested by Stian Skjelstad in a comment in:

http://openocd.zylin.com/#/c/2044/

if the USB product string cannot be read, provide a debug message so
users might get aware of a potential permission problem when looking
at the debug output.

Fix style bug found by Jenkins.

Change-Id: I6acb1c6261fec6f2bee80e4be513a5c5e29eff79
Signed-off-by: Jörg Wunsch <openocd@uriah.heep.sax.de>
Reviewed-on: http://openocd.zylin.com/2048
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>