There are machines out there which share legacy PCI IDE IRQs w/ other
devices.  libata SFF interrupt/HSM code is ready for shared IRQ and
has been setting IRQF_SHARED for devices in native PCI mode.  Device
in legacy mode is still a PCI device and thus supposedly uses
active-low level triggered IRQ.

Machines with such setup should be quite rare and w/o this flag libata
is likely to fail loading and render the system unuseable.  Also, IDE
driver has been setting IRQF_SHARED for devices in legacy mode for a
looooong time.
Signed-off-by: Tejun Heo <htejun@gmail.com>