cxl/hdm: Fix decoder count calculation

cxl/hdm: Fix decoder count calculation

The decoder count in the HDM decoder capability structure is an encoded
field. As defined in the spec:

Decoder Count: Reports the number of memory address decoders implemented
by the component.
0 – 1 Decoder
1 – 2 Decoders
2 – 4 Decoders
3 – 6 Decoders
4 – 8 Decoders
5 – 10 Decoders
All other values are reserved

Nothing is actually fixed by this as nothing actually used this mapping
yet.

Cc: Ira Weiny <ira.weiny@intel.com>
Fixes: 08422378c4ad ("cxl/pci: Add HDM decoder capabilities")
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
Link: https://lore.kernel.org/r/20210611190111.121295-1-ben.widawsky@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>