igb: remove redundant timer updates and cleanup watchdog_task

The igb watchdog task is modifying the watchdog timer twice duing a single
run.  It only needs to be called once to reschedule itself for 2 seconds from
the last time it ran.

In addition I removed the allocation of the mac_info structure since it is
only called twice and is easier to access via the e1000_hw struct.

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>