drivers: sensor: qdec_nrfx: Add missing REPORTRDY interrupt activation

This is a follow-up to commit 09fa46ee.

Before nrfx 3.0, the QDEC interrupt on REPORTRDY event was activated
implicitly when a `reportper` value other than `DISABLED` was used.
Now, the `reportper_inten` field needs to be used to activate this
interrupt.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>