iio: st_pressure: initialize lps22hb bootime

This patch initializes the bootime in struct st_sensor_settings for
lps22hb sensor. Without this, sensor channels read from sysfs always
report stale values.

Signed-off-by: Shrirang Bagul <shrirang.bagul@canonical.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>