dma-mapping: update comment after dmabounce removal

Commit e3217540

 ("ARM/dma-mapping: remove dmabounce") removes the
config DMABOUNCE. A comment to the function __dma_page_cpu_to_dev() refers
to this removed config DMABOUNCE.

Remove the obsolete explanation, but keep the recommendation not to use
__dma_page_cpu_to_dev() and use dma_sync_* functions instead.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>