spi: error status should be negative

Return a negative error value instead of a positive

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Cc: dmitry pervushin <dpervushin@embeddedalley.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>