kldxref: Refactor PNP entry parsing, no functional change

• Add a free_pnp_list to complement parse_pnp_list. Add freeing of 'new_desc' which was previously leaked.

• Move body of loop that checked a single pnp list element against a table entry into a parse_pnp_entry function to reduce indentation and split parse_entry into a smaller function.

• Similarly, split out a record_pnp_info function from parse_entry which builds the pnp_list and walks a table.

Reviewed by: imp
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D42965

(cherry picked from commit c40fa3dc98d3cd8c39605c19c9cc08026d8e72c9)