Fetch the current usb mode and return a string representation when
pyb.usb_mode() is called with no args. The possible string values are interned
as qstr's. None will be returned if an incorrect mode is set.
When setting usb_mode to "HID", hid config object now has
polling-interval (in ms) as the 4th element. It mmust now be a 5-tuple
of the form:
(subclass, protocol, max_packet_len, polling_interval, report_desc)
The mouse and keyboard defaults have polling interval at 8ms.
Apparently the order of interface numbers should be sequential and
increasing in a config descriptor. So as to retain compatibility with
Windows drivers for the CDC+MSC and CDC+HID modes, we move the CDC
configs to the end of the descriptors, instead of changing the interface
numbers.
See PR #957 for background.
