[Feature, netX51/50/10] Drv_Spi: Minimize/Eliminate pauses between 16 Byte chunks

Currently Drv_Spi blocks the data in 16 Byte chunks onto the wire (Implemented due to netX100/500 Errata Nr.9).

Sequence:
1. Fill FIFO
2. Start Transfer
3. Wait for FIFO Empty IRQ
4. goto 1

This results in pauses (measured 30us on netX50 running from SDRAM) between chunks