iio: humidity: hdc100x: correct humidity integration time mask

Apply the correct mask to enable all available humidity integration
times.  Currently, the driver defaults to 6500 and all is okay with that.
However, if 3850 is selected we get a stuck bit and can't change back
to 6500 or select 2500.  (Verified with HDC1008)

Signed-off-by: Alison Schofield <amsfield22@gmail.com>
Cc: Daniel Baluta <daniel.baluta@gmail.com>
Reviewed-by: Matt Ranostay <mranostay@gmail.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>