fbcon: check if the character count can be handled

Fontmaps can be 256 or 512 in length. The only driver that can do tileblitting
can only handle 256 characters.  Check for this when setting the font.

Signed-off-by: Antonino Daplas <adaplas@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>