ionic: Update driver to use ethtool_sprintf

Update the ionic driver to make use of ethtool_sprintf. In addition add
separate functions for Tx/Rx stats strings in order to reduce the total
amount of indenting needed in the driver code.

Acked-by: Shannon Nelson <snelson@pensando.io>
Signed-off-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>