staging: iio: remove duplicate case labels

Otherwise, compiling iio_event_monitor program fails:

io_event_monitor.c: In function ‘event_is_known’:
iio_event_monitor.c:125:2: error: duplicate case value
  case IIO_MOD_LIGHT_BOTH:
  ^
iio_event_monitor.c:121:2: error: previously used here
  case IIO_MOD_LIGHT_BOTH:

Fixes: da4db940 (iio staging: add recently added modifiers to iio_event_monitor)
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>