igb: minor type cleanups

This change cleans up some instances where unsigned int and u32
were being used interchangeably, and cleans up hdr_len which
was being set to 0 twice.

Signed-off-by: Nicholas Nunley <nicholasx.d.nunley@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>