usb: renesas_usbhs: fix NULL pointer dereference in dma_release_channel()

This patch fixes an issue that the following commit causes NULL
pointer dereference in dma_release_channel().
 "usb: renesas_usbhs: add support for requesting DT DMA"
 (commit id abd2dbf6)

The usbhsf_dma_init_dt() should set fifo->{t,r}x_chan to NULL if
dma_request_slave_channel_reason() returns IS_ERR value.
Otherwise, usbhsf_dma_quit() will call dma_release_channel(), and then
NULL pointer dereference happens.

Acked-by: Geert Uytterhoeven <geert@linux-m68k.org>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Reported-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>