bna: Update driver to use ethtool_sprintf

Update the bnad_get_strings to make use of ethtool_sprintf and avoid
unnecessary line wrapping. To do this we invert the logic for the string
set test and instead exit immediately if we are not working with the stats
strings. In addition the function is broken up into subfunctions for each
area so that we can simply call ethtool_sprintf once for each string in a
given subsection.

Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>