spi: atmel_spi update chipselect handling

This solves several issues:
  * It fixes the wrong idle clock polarity issue in a cleaner and less
    expensive way.
  * It handles the AT32AP7000 errata "SPI Chip Select 0 BITS field
    overrides other Chip Selects". Other chips, e.g. AT91SAM9261, have
    similar issues.

Currently, the AT91RM9200 code path is left alone. But it might be
interesting to try the same technique on RM9200 using a different CSR
register.

[dbrownell@users.sourceforge.net: restore debug message for activation]
Signed-off-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>