fx2lafw: add support for "probe names" scan option

fx2lafw: add support for "probe names" scan option

Add support for the SR_CONF_PROBE_NAMES scan option. Optionally assign
user specified channel names instead of the driver's builtin names.

It's essential to create sdi and devc before scanning the probe names
spec. The previous logic which printed channel names to a local buffer
got replaced by a const table of channel names. This commit is a little
longer because checks are added, to make sure that future models with
more channels won't exceed the static const tables.

This implementation exclusively allows to specify logic channel names.
Analog channels keep the open coded naming pattern. Because the driver
supports different models with differing numbers of channels of several
types, covering analog channels in the "probe names" feature requires
more runtime effort to create the table of default channel names for the
specific device that was found. This remains as an option for a later
commit.