scpi-pps: don't fail acquisition start for devices with no CMD_SELECT_CHANNEL

scpi-pps: don't fail acquisition start for devices with no CMD_SELECT_CHANNEL

For devices such as the HP 6632B the following invocation was failing due to
scpi_cmd(sdi, SCPI_CMD_SELECT_CHANNEL, ...) returning SR_OK_CONTINUE.

./sigrok-cli -d scpi-pps:conn=/dev/ttyUSB0:serialcomm=9600/8n1 --continuous
sr: session: sr_session_start: could not start an acquisition (not enough data to decide error status yet)
Failed to start session.