drivers: sensors: fix DRDY usage

hts221_trigger.c is pulled in by the CMakeLists.txt file when
CONFIG_HTS221_TRIGGER is defined. Therefore make the source in hts221.c use
the some define.

This removes also the (wrong) warning

    <inf> HTS221: Cannot enable trigger without drdy-gpios

even when "drdy-gpios" is defined in the device-tree.

Signed-off-by: Holger Schurig <holgerschurig@gmail.com>