htu21: don't needlessly bother hardware when measurements are not needed

sysctl(8) first queries a sysctl to get a size of its value even if the
sysctl is of a fixed size, e.g. it has an integer type.
Only after that sysctl(8) queries an actual value of the sysctl.

Previosuly the driver would needlessly read a sensor in the first step.

MFC after: 1 week