Build fixes with missing qmux support in libmbim

We have a few bits of unprotected conditional code that fails to build if libqmi does not support qmux.

Some of the checks also do not use definedness for the internal WITH_QMI symbol, but just check its value; if WITH_QMI is set to 1, this will shortcircuit the other side of the pre-processor conditional, and the QMI_MBIM_QMUX_SUPPORTED symbol will never be checked.

Fixes: #84 (closed)