nvme: invalidate correct memory range after read

The current code invalidates the range after the read buffer since the
buffer pointer gets incremented in the read loop. Use a temporary
pointer to make sure we have a pristine pointer to invalidate the
correct memory range after read.

Fixes: 704e040a ("nvme: Apply cache operations on the DMA buffers")
Reviewed-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Stefan Agner <stefan@agner.ch>