HID: intel-ish-hid: Move DMA disable code to new function

Add a new function ish_disable_dma() and move DMA disable operations
here, so that this functionality can be reused.

Signed-off-by: Even Xu <even.xu@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>