ethtool: netlink: Slightly simplify 'ethnl_features_to_bitmap()'

ethtool: netlink: Slightly simplify 'ethnl_features_to_bitmap()'

The 'dest' bitmap is fully initialized by the 'for' loop, so there is no
need to explicitly reset it.

This also makes this function in line with 'ethnl_features_to_bitmap32()'
which does not clear the destination before writing it.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/17fca158231c6f03689bd891254f0dd1f4e84cb8.1638091829.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jakub Kicinski <kuba@kernel.org>