ixgbe/base: power down the X550em PHY on overtemp event

ixgbe/base: power down the X550em PHY on overtemp event

This patch powers down the x550em PHY on over-temp events. The
PHY firmware is supposed to do this autonomously but that isn't
implemented. The short-term stop-gap solution is for SW to power
down the PHY when it reports an overtemp event.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>