devices: read the actual operating mode when the device is in AT mode

[diescalo]

• Completed the list of operating modes.
• If the device cannot be discovered in API mode, the library checks if it is in AT mode by entering command mode. In some scenarios like a hardware reset or recovery mode, the module could be in API mode, but the library cannot find it and determines that it is in AT mode (raising an exception). To avoid that, the library now asks for the actual operating mode after entering command mode.
• Added a new method to exit command mode.

Signed-off-by: Diego Escalona diego.escalona@digi.com