The synapticsmst-common.c file had some global state so that cascade devices
could use the device fd. This made the control flow error prone, and it meant
that the fd could be leaked trivially on any error path.
Moving the fd ownership to the device is the logical place for the control, and
means we can create "connections" to access the main device and the cascade
devices.
This fixes the warnings detected by Coverity.
