ixgbe: Add const string for overheat message

The over heat message is displayed for multiple reasons but the text is
cut-n-pasted for each of these cases.  This patch pulls the text from the
same location.  I noticed this as I am about to add another case.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>