sam3 spi: use data transfer register interrupt instead of tx finished interrupt:...

sam3 spi: use data transfer register interrupt instead of tx finished interrupt: this
way transfer is faster and we avoid chip select deasserting after each byte of a multi-byte
transfer.
Set all chip select registers at initialization, in case user change chip select manually
(at the moment the spi driver doesn't have an API to change chip select).

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4442 38d2e660-2303-0410-9eaa-f027e97ec537