ehea: fix delayed packet processing

In the eHEA poll function an rmb() is required. Without that some packets
on the receive queue are not seen and thus delayed until the next interrupt
is handled for the same receive queue.

Signed-off-by: Jan-Bernd Themann <themann@de.ibm.com>
Signed-off-by: Jay Vosburgh <fubar@us.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>