media: ipu6: make the ipu6_mmu_unmap() as a void function

The DMA unmap API is not supposed to return value. Thus this patch
changes the ipu6_mmu_unmap() as a void function and DMA unmapping
didn't check the return value.

Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
[Sakari Ailus: Drop unnecessary returns.]
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>