Apart from the standard "configurations", "interfaces" and "alternate
interface settings" in USB, iOS devices also have a notion of
"modes". In different modes, the device exposes a different set of
available configurations.
Depending on the iOS version, and depending on the current mode, the
length and contents of the carrier state control message differs:
* 1 byte (seen on iOS 4.2.1, 8.4):
* 03: carrier off (mode 0)
* 04: carrier on (mode 0)
* 3 bytes (seen on iOS 10.3.4, 15.7.6):
* 03 03 03: carrier off (mode 0)
* 04 04 03: carrier on (mode 0)
* 4 bytes (seen on iOS 16.5, 17.6):
* 03 03 03 00: carrier off (mode 0)
* 04 03 03 00: carrier off (mode 1)
* 06 03 03 00: carrier off (mode 4)
* 04 04 03 04: carrier on (mode 0 and 1)
* 06 04 03 04: carrier on (mode 4)
Before this change, the driver always used the first byte of the
response to determine carrier state.
