Add hw.usb.no_pf="1" to loader.conf on nanobsd

Add hw.usb.no_pf="1" to the default loader.conf file on wrap / nanobsd.
This should also fix the problem discussed here https://github.com/bsdperimeter/pfsense/commit/25c088de57be43155d7f7df202f800016d5f515d where the setting is not written to the loader.conf file when upgrading from an old version.