lsr/es51919: support channel selection (enable/disable P1/P2)

lsr/es51919: support channel selection (enable/disable P1/P2)

Upon reception of serial data from the ES51919 LCR chipset, the data for
channels P1 and P2 was extracted from the packet, and unconditionally got
sent to the sigrok session.

Do check the channels' enabled state before submission. This fixes for
serial-lcr what recently got reported for a Brymen DMM. Tested with

  $ sigrok-cli -d peaktech-2170:conn=/dev/ttyUSB0 --channels P2

and other --channels specifications.