net/nfp: use generic PCI config access functions

net/nfp: use generic PCI config access functions

This patch avoids direct access to device config sysfs file using
rte_pci_read_config instead.

Apart from replicating code, it turns out this direct access does
not always work if non-root users execute DPDK apps. In those cases
it is mandatory to go through VFIO specific function for reading pci
config space.

Signed-off-by: Alejandro Lucero <alejandro.lucero@netronome.com>