drivers: sensor: qdec_nrfx: Store sensor trigger as a pointer

Fixes the qdec_nrfx sensor driver to store the user-supplied sensor
trigger as a pointer rather than a copy. This enables the trigger
handler to use CONTAINER_OF to retrieve a context pointer when the
trigger is embedded in a larger struct.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>