HID: intel-ish-hid: ishtp: add helper functions for client buffer operation

Add helper ishtp_cl_tx_empty() and ishtp_cl_rx_get_rb() to hide internal
details from callers, who needs this functionality.

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>