ixgbe: Refactor overtemp event handling

Currently ixgbe driver is notified of overheating events
via internal IXGBE_ERR_OVERTEMP error code.

Change the approach for handle_lasi() to use freshly introduced
is_overtemp function parameter which set when such event occurs.
Change check_overtemp() to bool and return true if overtemp
event occurs.

Reviewed-by: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Signed-off-by: Jedrzej Jagielski <jedrzej.jagielski@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Tested-by: Sunitha Mekala <sunithax.d.mekala@intel.com> (A Contingent worker at Intel)
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>