nfp: remove unused parameter from nfp_net_write_mac_addr()

nfp_net_write_mac_addr() always writes to the BAR the current
device address taken from netdev struct.  The address given
as parameter is actually ignored.  Since all callers pass
netdev->dev_addr simply remove the parameter.

While at it improve the function's kdoc a bit.

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