auxdisplay: ht16k33: Use HT16K33_FB_SIZE in ht16k33_initialize()

Use the existing HT16K33_FB_SIZE definition instead of open-coding the
same calculation using an hardcoded value.
While at it, restore reverse Christmas tree variable declaration order.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Robin van der Gracht <robin@protonic.nl>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>