igc: Update driver to use ethtool_sprintf

Complete to commit c8d4725e ("intel: Update drivers to use
ethtool_sprintf")
Update the igc driver to make use of ethtool_sprintf. The general idea
is to reduce code size and overhead by replacing the repeated pattern of
string printf statements and ETH_STRING_LEN counter increments.

Suggested-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Tested-by: Dvora Fuxbrumer <dvorax.fuxbrumer@linux.intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>