PX4IO: re-enable handling digital rssi

The px4io driver was ignoring the contents of PX4IO_P_RAW_RC_NRSSI
since digital rssi wasn't working properly. This change was made in
commit 41b127d4.

Now that PX4IO_P_RAW_RC_NRSSI is returning valid values for digital
rssi, we can re-enable its use.