mt76: debugfs: fix queue reporting for mt76-usb

Fix number of rx-queued frames reported by mt76_usb driver.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: 2d8be76c ("mt76: debugfs: improve queue node readability")
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>