usbnet: ipheth: fix carrier detection in modes 1 and 4

usbnet: ipheth: fix carrier detection in modes 1 and 4

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.

From this larger sample, the first byte seems to indicate the number of
available USB configurations in the current mode (with the exception of
the default mode 0), and in some cases (namely mode 1 and 4) does not
correlate with the carrier state.

Previous logic erroneously counted `04 03 03 00` as "carrier on" and
`06 04 03 04` as "carrier off" on iOS versions that support mode 1 and
mode 4 respectively.

Only modes 0, 1 and 4 expose the USB Ethernet interfaces necessary for
the ipheth driver.

Check the second byte of the control message where possible, and fall
back to checking the first byte on older iOS versions.

Signed-off-by: Foster Snowhill <forst@pen.gy>
Tested-by: Georgi Valkov <gvalkov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>