spi: simplify spi_sync() calling convention

Simplify spi_sync calling convention, eliminating the need to check both
the return value AND the message->status.  In consequence, this corrects
misbehaviours of spi_read and spi_write (which only checked the former) and
their callers.

Signed-off-by: Marc Pignat <marc.pignat@hevs.ch>
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>