qlge: Fix MSI/legacy single interrupt bug.

The chip can issue spurious interrupts for single interrupt
modes. We use disable to clear the condition and allow processing to
continue. Also got rid of legacy specific code since it now needs to
be done on MSI single irq also.

Signed-off-by: Ron Mercer <ron.mercer@qlogic.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>