HID: intel-ish-hid: ishtp: add helper function for client search

Add helper function ishtp_fw_cl_get_client() for client driver searching
client information to hide internal details from callers.

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>