ixgbe: cleanup ATR filter setup function

This change cleans up the ixgbe_atr filter setup function so that it uses
fewer items from the stack.  Since the code is only applicable to IPv4 w/
TCP it makes sense to just use the pointers based on the headers themselves
instead of copying them to temp variables and then writing those to the
filters.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Tested-by: Ross Brattain <ross.b.brattain@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>