Velocity warning before receiver clock is set

[ljbade]

After https://github.com/swift-nav/piksi_firmware/pull/651lands there is a message:
velocity > 1000kts
during the receiver startup.

This is because calc_PVT always spits out warning, even before the receiver clock is set and velocity becomes valid. The actual result is not used by the solution thread for positioning, only to set the receiver clock, so never makes it into SBP.

A solution might be to add a parameter to calc_PVT that will suppress warnings before the receiver clock is set.