fonts: Use BUILD_BUG_ON() for checking empty font table

We have a nice macro, and the check of emptiness of the font table can
be done in a simpler way.

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>