sam3n spi: revert to "end of transmit" interrupt instead of "tx buffer empty",

sam3n spi: revert to "end of transmit" interrupt instead of "tx buffer empty",
it's slower but with the latter method it's necessary to handle both interrupt
to make flush work correctly.
Remove board specific stuff (chip select handling).  Don't allow SPI hardware
to drive chip select, better handled by the app as PIO.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4447 38d2e660-2303-0410-9eaa-f027e97ec537