usb: renesas_usbhs: fifo: request DMAEngine once

DMAEngine uses IRQ if dma_request_channel() was called,
and it is using devm_request_irq() today,
OTOH, dma_request_channel() will be called when each
USB connection happened on this driver.
This means same IRQ will be requested many times
whenever each USB connected,
and this IRQ isn't freed when USB disconnected.
Request DMAEngine once.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>