UBX _buf.payload_rx_nav_pvt.nano can become negative

[PetervdPerk-NXP]

In my setup using an Pixhawk 4 GPS, sometimes the nanoseconds in the UBX_MSG_NAV_PVT can become negative.

This is not a severe problem since PX4-Firmware doesn't use nanoseconds since CONFIG_RTC_HIRES is disabled thus it only uses the epoch in seconds.