HID: lg-g15: Add a lg_g15_init_input_dev() helper function

Factor the input-device setup + KEY_KBD_LCD_MENU capability setting out
of lg_g15_probe() into a new lg_g15_init_input_dev() helper function.

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>