3c59x: Fix shared IRQ handling

As its first order of business, boomerang_interrupt() checks whether
the device really has any pending interrupts. If it does not,
it does nothing and returns, but it still returns IRQ_HANDLED.

This is wrong: interrupt was not handled, IRQ handlers of other
devices sharing this IRQ line need to be called.

vortex_interrupt() has it right: it returns IRQ_NONE in this case
via IRQ_RETVAL(0).

Do the same in boomerang_interrupt().

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
CC: David S. Miller <davem@davemloft.net>
CC: linux-kernel@vger.kernel.org
CC: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>