igb: simplify swap in clean_rx_irq if using packet split

This update replaces the xchg calls that were added with a pair of
assignments as there is no need for the xchg calls and they were found to
cause issues on some architectures.

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>