greatfet: support capture of the upper pin bank (first pin 8)

greatfet: support capture of the upper pin bank (first pin 8)

Present all 16 channels to users. By default lower channels are enabled
and upper channels are disabled. Support the configuration where users
capture upper channels after disabling lower channels. Device firmware
provides the "first pin" feature and only communicates pin states for
the upper bank to the host.

This mode of operation can be useful to inspect different signal sets
without physically moving probes. Upper bank capture combines well with
higher rates at lower pin counts.

This implementation in theory also covers 16 channel captures, though
the firmware does not officially support multi-bank captures. Emit a
warning when this setup gets detected, but don't refuse operation and
attempt a best effort capture.

This commit also improves reliability in paths where failed acquisition
starts shutdown the operation, before another request gets serviced.