enumeration: Refactor discover sensors code

enumeration: Refactor discover sensors code

The functions for discovering poll sensors (discover_poll_sensors)
and trigger sensors (discover_trig_sensors) contain duplicate
code. If more sensor types need to be discovered (e.g. event sensors),
this would add even more duplicate code.

Refactor these two functions to minimize code duplication.

Change-Id: I92ac6f8c6a54afbce5a3ffb45d4b011204e8c754
Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Daniel Baluta <daniel.baluta@intel.com>