igb: add additional error handling to the phy code

This update adds additional exception handling to the phy code to handle
situations where it may be called incorrectly.  In addition it adds some
bounds checking to the cable length checks to prevent an array overrun in
the event that the hardware returned a different value than expected.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>