smsc9420: fix interrupt signalling test failures

smsc9420 performs an interrupt signalling test when the interface is
brought up.  The current code mistakenly sets its test flag to false
AFTER enabling the software interrupt source, making failure quite
likely.

This patch changes the code to set the test flag BEFORE enabling
interrupts. I've also removed an smp_wmb because the following spinlock
provides an implicit memory barrier.

Signed-off-by: Steve Glendinning <steve.glendinning@smsc.com>
Signed-off-by: David S. Miller <davem@davemloft.net>