serial-dmm: bm86x: increase packet request frequency

serial-dmm: bm86x: increase packet request frequency

Request packets from the Brymen BM86x meter much faster. The previous
implementation in the separate driver used to immediately send another
request when a measurement arrived, with a 10ms granularity in the poll
routine, and a 500ms timeout between requests.

Considering the meter's update rate, stick with the 500ms timeout, but
increase the maximum request rate to 10 per second, with a minimum of 2
per second. This receives measurement data at the meter's capability
(compare DC and AC modes, seems to automatically adjust to the internal
operation, and match the display update rate).