wl18xx: deprecate PG1 support

The new PG2 version of the chip has a few differences in terms of FW
API if compared to PG1.  PG1 is just a sample that shouldn't be used
in real life, so to avoid having to handle both separately, mark the
PG1 version as deprecated and bail out during probe.

Signed-off-by: Luciano Coelho <coelho@ti.com>