ICM_20948_Device_t with _last_bank set to an invalid number

[peterleif]

Resolves #71

• Implement ICM_20948_init_struct function
• Initialize _last_bank to 4 (invalid bank number)
• Update C++ constructor to use ICM_20948_init_struct
• Update the Example999

[PaulZC]

Thank you Etsushi (@peterleif), I will test your code and then add this to the next release. Best wishes, Paul