linux-phytec-ti

usb: dwc2: hcd: fix build warning

commit db62b9a804b4 (usb: dwc2: host: don't
use dma_alloc_coherent with irqs disabled)
introduced a build warning by using NULL
as an integer. Fix that by just using 0 instead
of NULL.

Signed-off-by: Felipe Balbi <balbi@ti.com>