Optimized CS8900A driver for speed and size.

- Speed: The primary byte copy loops are reduzed to the bare minimum by adjusting the base pointer 'ptr' and loop register 'y' in such a way that the 'y' overflow matches the low byte of the loop size.

- Size: Factored out all repeated code into subroutines. Introduced a loop for setting the MAC address.

Additional minor changes:

- Activate frame reception as last step of initialization after CS8900A configuration.

- Properly set internal address bits used by the CS8900A.