IAR generates warning Pe111 'statement is unreachable'. In a couple of
cases, replace return statements with TU_ATTR_FALLTHROUGH; because some
compilers apparently can't figure out that the return statements are
unreachable but do whinge about an imagined fall-through without them!
- add tud_hid_n_interface_protocol()
- rename tud_hid_n_boot_mode() to tud_hid_n_get_protocol()
- rename tud_hid_boot_mode_cb() to tud_hid_set_protocol_cb()
- add HID_PROTOCOL_BOOT/REPORT to avoid magic number 0,1
- rename HID_PROTOCOL_NONE/KEYBOARD/MOUSE to HID_ITF_PROTOCOL_ to avoid
confusion
