ATA convert GSI to irq on ia64

If an ATA drive uses legacy mode, ata driver will choose 14 and 15
as the fixed irq number. On ia64 platform, such numbers are GSI and
should be converted to irq vector.

Below patch against kernel 2.6.20 fixes it.

Signed-off-by: Zhang Yanmin <yanmin.zhang@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>