HID: intel-ish-hid: use helper function to access client buffer

HID: intel-ish-hid: use helper function to access client buffer

ishtp bus driver exposed helper functions for client buffer accessing,
so change to use these functions in ishtp-hid-client driver to avoid
access client buffer directly.

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