Static analysis with CoverityScan is throwing warnings that specific
case statements are missing breaks.  Rather than adding breaks, add
return -EINVAL to the specific case statements to clarify the
error return paths. Fix also saves 50 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  21418	   4936	    128	  26482	   6772	drivers/iio/adc/ina2xx-adc.o

After:
  dec	    hex	filename
  21370	   4936	    128	  26434	   6742	drivers/iio/adc/ina2xx-adc.o

(gcc 8.2, x86-64)

Detected by CoverityScan, CID#1462408 ("Missing break in switch")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>