usb: dwc2: fix debugfs FIFO count

The number of FIFOs may be lower than the number of endpoints.  Use the
correct total when printing FIFO details in debugfs.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: John Keeping <john@metanate.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>