ixgbe: cleanup string function calls to use bound checking versions.

Some minor cleanup to use string calls that use bound checks just to
be extra safe.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Stephen Ko <stephen.s.ko@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>