usb: renesas: fix scheduling in atomic context bug

The current renesas_usbhs driver triggers

BUG: scheduling while atomic: ksoftirqd/0/3/0x00000102

with enabled CONFIG_DEBUG_ATOMIC_SLEEP, by submitting DMA transfers from
an atomic (tasklet) context, which is not supported by the shdma dmaengine
driver. Fix it by switching to a work.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>