hp-59306a: only match when conn= is given to avoid false probe positives

The HP 59306A device was made in 1973 and would not reliably identify
by means of SCPI queries. The previous scan() implementation would have
matched any connected SCPI device, then upset these devices by emitting
non-SCPI requests.

Tighten the scope of the hp-59306a probe, only scan for devices when a
conn= spec was provided. This avoids false positives and malfunction of
other devices, and still lets users address the problematic device. This
is similar to serial-dmm and unspecific cables and has proven to work
there.

How to reproduce the issue:

  (while any auto enumerating SCPI device is connected)
  $ sigrok-cli --scan