HID: intel_ish-hid: Enhance API to get ring buffer sizes

Added two APIs:

- ishtp_cl_get_tx_free_buffer_size:
This returns total size available for a client to queue TX data.

- ishtp_cl_get_tx_free_rings:
This returns total number of TX rings available for a client.

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