nfp: move rtsym helpers to pf code

nfp_net_pf_rtsym_read_optional() and nfp_net_pf_map_rtsym() are not
really related to networking code.  Move them to the PF code and
remove the net from their names.  They will soon be needed by code
outside of nfp_net_main.c anyway.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>