trivial: atmel_lcdfb: add missing error message

When a too small framebuffer is given, the atmel_lcdfb_check_var
silently fails.
Adding an error message will save some head scratching.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>