libusb: add debug message on adapter not found due to wrong serial

libusb: add debug message on adapter not found due to wrong serial

When few adapters of the same type are in use, the serial string is
the way to select the right one.
Currently a serial string that does not match any of the connected
adapters will just fail the open, without specific information to
track the issue.

Add a specific message to highlight that the open failure is caused
by a serial mismatch.

Change-Id: I5cb77f1045cc746e532d395b2e5ced40a23ab638
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4701
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>