Constants for CXL capability should not change

Constants for CXL capability should not change

When CXL capability decoding was upgraded to revision 2 by commit
c0ccce1b4cd5b42b17f2e8f7bae4031c311677ff, the value of PCI_CXL_DEV_LEN
in lib/header.h has changed.

This is probably not a good idea - programs using libpci can depend
on the exact value of this constant.

Let us revert PCI_CXL_DEV_LEN to the original value for revision 1
and add PCI_CXL_DEV_LEN_REV2 for the next revision.

Also, fixed a bug in the decoder which caused it to read past the
end of the buffer for a capability which is declared as revision 2,
but too short.