tdfxfb: fix frame buffer name overrun

If there are more then one graphics card handled by the tdfxfb driver the
name of the frame buffer overruns reserved size.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>