HID: lg-g15: Make the LED-name used by lg_g15_register_led() a parameter

Make the LED-name used by lg_g15_register_led() a parameter.

This is a preparation patch for adding support for the LCD menu
keys + LCD brightness control on the Logitech Z-10 speakers (with LCD)
which use the same protocol as the G15 keyboards.

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