staging: gasket: fix data page unmap DMA direction

The DMA direction supplied to dma_unmap_page should match the
corresponding dma_map_page call, which is mapped bi-directional.

Signed-off-by: Nick Ewalt <nicholasewalt@google.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>