net/nfp: fix setting MAC address

Some firmwares, mostly for VFs, do not advertise the feature /
capability of changing the MAC address while the interface is up. With
such firmware a request to change the MAC address that at the same
time also tries to enable the not available feature will be denied by
the firmware resulting in an error message like:

nfp_net_reconfig(): Error nfp_net reconfig for ctrl: 80000000 update: 800

Fix set_mac_addr by not trying to enable a feature if it is not
advertised by the firmware.

Fixes: 2fe669f4bcd2 ("net/nfp: support MAC address change")
Cc: stable@dpdk.org
Signed-off-by: Pablo Cascón <pablo.cascon@netronome.com>
Acked-by: Alejandro Lucero <alejandro.lucero@netronome.com>