iio: pressure: bmp280: fix relative humidity unit

According to IIO ABI relative humidity reading should be
returned in milli percent.

This patch addresses that by applying proper scaling and
returning integer instead of fractional format type specifier.

Note that the fixes tag is before the driver was heavily refactored
to introduce spi support, so the patch won't apply that far back.

Signed-off-by: Tomasz Duszynski <tduszyns@gmail.com>
Fixes: 14beaa8f

 ("iio: pressure: bmp280: add humidity support")
Acked-by: Matt Ranostay <matt.ranostay@konsulko.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>