staging: iio: accel: fix error check

commit ef3149eb upstream.

sca3000_read_ctrl_reg() returns a negative number on failure, check for
this instead of zero.

Signed-off-by: Luis de Bethencourt <luisbg@osg.samsung.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>