ixgbe: minor sparse fixes

Make strings const if possible, and fix includes so forward definitions
are seen.

Signed-off-by: Stephen Hemminger <shemminger@linux-foundation.org>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>