igb: update hw_debug macro to make use of netdev_dbg call

This change updates the igb driver to make use of the netdev_dbg function
macros now provided in netdevice.h

This is meant to be provided as an alternative to the patch provided by
Joe Perches.

It also removes igb_get_time_str since I found that it is unused code that
is no longer used even in debug.

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>