HID: ishtp-hid-client: Fix 'suggest-attribute=format' compiler warning

Fixes the following W=1 kernel build warning(s):

 drivers/hid/intel-ish-hid/ishtp/bus.c: In function ‘ishtp_trace_callback’:
 drivers/hid/intel-ish-hid/ishtp/bus.c:876:29: warning: return type might be a candidate for a format attribute [-Wsuggest-attribute=format]
 876 | return cl_device->ishtp_dev->print_log;
 | ~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~

Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: Daniel Drubin <daniel.drubin@intel.com>
Cc: linux-input@vger.kernel.org
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>