HID: lg-g15: Add a lg_g15_handle_lcd_menu_keys() helper function

Factor out the handling of the G15 LCD menu keys out of
lg_g15_event() into a new lg_g15_handle_lcd_menu_keys() helper function.

This is a preparation patch for adding support for the LCD menu
keys on the Logitech Z-10 speakers (with LCD) which use the same
funky HID report format.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>