iOS-only: Use maximum output channel count for interface

[paulreimer]

Allows using (cheap) 5.1 / 7.1 channel interfaces as a 6-channel / 8-channel interface instead of the default stereo behaviour for such interfaces on iOS.

Example product: https://www.amazon.ca/gp/product/B07QBZVNPW

Fulfills https://github.com/sonosaurus/sonobus/issues/137