HID: i2c-hid: note that I2C xfer buffers are DMA-safe

All I2C communications in the driver use driver-private buffers that are
DMA-safe, so mark them as such.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Tested-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>