Hard coded for all PORTD row bits being zero

Used a #define to eliminate some code when all port d row bits are zero
(which they happen to be now. Saved 26 bytes.
Fixed last commit

A little overagressive and missed setting PORTD to 0 in one case. Added
back 2 bytes.
Cleaned up LED code and fixed brightness test

Made the LED code very simple and clean asm() with very tight loop. Also
finally got the brightness test working - turns out that in C 2^5 is not
equal to 32! :)