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

The unhandled bits case was highlighted by smatch:
  CHECK   drivers/iio/industrialio-core.c
drivers/iio/industrialio-core.c:719 iio_device_add_info_mask_type() error: buffer overflow 'iio_chan_info_postfix' 17 <= 31
  CC [M]  drivers/iio/industrialio-core.o
  CHECK   drivers/iio/industrialio-event.c
drivers/iio/industrialio-event.c:327 iio_device_add_event() error: buffer overflow 'iio_ev_info_text' 3 <= 3

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.

Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Cc: Lars-Peter Clausen <lars@metafoo.de>
Cc: Dan Carpenter <dan.carpenter@oracle.com>