USB: fix USBTMC get_capabilities success handling

In order:
Add reference to relevant section of USBTMC usb488 subclass specs.
Print debug output of capabilities only when it was retrieved successfully.
Clear return value on success, otherwise driver always reports failure.

Signed-off-by: Gergely Imreh <imrehg@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>