spi: spi_bfin cleanups, error handling

Cleanup and error handling

 - add error handling in SPI bus driver with selecting clients
 - use proper defines to access Blackfin MMRs
 - remove useless SSYNCs
 - cleaner use of portmux calls

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.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>