spi_mpc83xx: reject invalid transfer sizes

Error out on transfer length != multiple of bytes per word with -EINVAL.
Fixes a buffer overrun crash if length < bytes per word.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>
Acked-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
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>