iio:core: Fix bug in length of event info_mask and catch unhandled bits set in masks.

The incorrect limit for the for_each_set_bit loop was noticed whilst fixing
this other case.  Note that as we only have 3 possible entries a the moment
and the value was set to 4, the bug would not have any effect currently.
It will bite fairly soon though, so best fix it now.

See commit ef4b4856 ("iio:core: Fix bug in length of event info_mask and
catch unhandled bits set in masks.") for details.

Signed-off-by: Young Xiao <92siuyang@gmail.com>
Reviewed-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>