video: fbdev: atmel: fix warning for const return value

A const on a return value is meaningless and generates a warning on some
versions of gcc:

drivers/video/fbdev/atmel_lcdfb.c:1003: warning: type qualifiers ignored on function return type

The function in question is only used inside the .c file, so the author
of the code most likely means "static" instead of "const".

Change the const to static.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>