nfp: Replace nfp_pr_et with ethtool_sprintf

The nfp_pr_et function is nearly identical to ethtool_sprintf except for
the fact that it passes the pointer by value and as a return whereas
ethtool_sprintf passes it as a pointer.

Since they are so close just update nfp to make use of ethtool_sprintf

Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>