iwlwifi: local namespace change for sysctl

In order for the FreeBSD-specific sysctl (which gives us a full list of
PCI device IDs/names/and firmware prefixes) to be in the proper namespace
add a LINUXKPI_PARAM_PREFIX definition to the file as well.

The sysctl can now be used as:
sysctl -n compat.linuxkpi.iwlwifi_pci_ids_name

Sponsored by: The FreeBSD Foundation
MFC after: 3 days