Need to check size of session channel mask!

When we added channel shuffle map save/restore support (377f066) we didn't realize that this changed the layout of the US version of the session channel mask. This causes compatibility problems when upgrading images built in the interim between 6158a1c and 377f066.

The appropriate thing is to validate the size of the ChannelMask using the header in `Arduino_LoRaWAN::ApplySessionState()`.