auxdisplay: charlcd: simplify init message display

If CONFIG_PANEL_CHANGE_MESSAGE is set, CONFIG_PANEL_BOOT_MESSAGE will
also be defined, so the double ifdef is pointless.  Simplify the code
further by using an intermediate macro rather duplicating most of the
line.

Signed-off-by: Mans Rullgard <mans@mansr.com>
Signed-off-by: Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>