HID: intel-ish-hid: Remove unused ishtp_dev_state_str

ishtp_dev_state_str() was added in 2016 by
commit 3703f53b ("HID: intel_ish-hid: ISH Transport layer")
but has never been used.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>