drivers: sensors: apds9960 fix trigger callback context

In the current implementation, the apds9960 trigger callback function
is called with the pointer of a driver-internal allocated trigger
structure. This structure is not initialized anywhere, so essentially
the trigger callback gets called with junk data.

Besides the missing initialization, it would be better instead to hold
a const pointer to a user allocated sensor_trigger object.
This way user code can establish a context with related user data (for
example a pointer to a C++ object) by storing its sensor_trigger object
within a structure alongside the user data, and then using
CONTAINER_OF() macro to get the pointer of the container struct (and
thus the user data).

Signed-off-by: Michael Kaplan <m.kaplan@evva.com>