sh: Quiet noisy coherent DMA unmapping on R2D.

IRQs are re-enabled at a later stage when doing the unmapping on R2D via
the sm501 USB coherent DMA, resulting in the irqs_disabled() check
producing considerable noise for this configuration. Just kill off the
check, which was blindly copied from x86 anyways.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>