MAINTAINERS: Update SWIOTLB maintainership

Konrad's new job role is putting a serious cramp on him
being a responsive maintainer and as such he is handing off
the reins to Christoph Hellwig.

Thank you!

Acked-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>