scd: Add a libusb debug level.

* scd/ccid-driver.c (USE_LIBUSB_DEBUG_CB): New const.
(debug_libusb_cb): new.
(ccid_set_debug_level): Use it.

This allows to see the libusb log in our usual debug output. For this
the option debug-ccid-driver needs to be given 5 or more times.