HID: hid-logitech-hidpp: Fix conformant kernel-doc header and demote abuses

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

 drivers/hid/hid-logitech-hidpp.c:275: warning: Function parameter or member 'hidpp' not described in 'hidpp_send_message_sync'
 drivers/hid/hid-logitech-hidpp.c:275: warning: Function parameter or member 'message' not described in 'hidpp_send_message_sync'
 drivers/hid/hid-logitech-hidpp.c:275: warning: Function parameter or member 'response' not described in 'hidpp_send_message_sync'
 drivers/hid/hid-logitech-hidpp.c:275: warning: expecting prototype for and something else(). Prototype was for hidpp_send_message_sync() instead
 drivers/hid/hid-logitech-hidpp.c:427: warning: wrong kernel-doc identifier on line:
 drivers/hid/hid-logitech-hidpp.c:470: warning: Function parameter or member 'input_dev' not described in 'hidpp_scroll_counter_handle_scroll'
 drivers/hid/hid-logitech-hidpp.c:1894: warning: Function parameter or member 'hidpp' not described in 'hidpp_touchpad_fw_items_set'
 drivers/hid/hid-logitech-hidpp.c:1894: warning: Function parameter or member 'feature_index' not described in 'hidpp_touchpad_fw_items_set'
 drivers/hid/hid-logitech-hidpp.c:1894: warning: Function parameter or member 'items' not described in 'hidpp_touchpad_fw_items_set'
 drivers/hid/hid-logitech-hidpp.c:1894: warning: expecting prototype for send a set state command to the device by reading the current items(). Prototype was for hidpp_touchpad_fw_items_set() instead

Cc: Jiri Kosina <jikos@kernel.org>
Cc: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Cc: Henrik Rydberg <rydberg@bitmath.org>
Cc: Lopez Casado <nlopezcasad@logitech.com>
Cc: linux-input@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>