Staging: iio: signedness bug

i2c_smbus_read_byte_data() returns an s32 type.  We need to change
"rate" to signed for the error handling to work.

Also I changed it to propogate the error code instead of just returning
-EINVAL.  Other error codes could be -EAGAIN for example.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>