staging: unisys: Fix sparse error - accessing __iomem directly

Copy the channel type into a temporary buffer so that code will work
for architectures that don't support MMIO. This now works in same way
as other tests in same function.

Signed-off-by: Luke Hart <luke.hart@birchleys.eu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>