AHCI: Do not read HOST_IRQ_STAT reg in multi-MSI mode

As described in AHCI v1.0 specification chapter 10.6.2.2
"Multiple MSI Based Messages" generation of interrupts
is not controlled through the HOST_IRQ_STAT register.

Considering MMIO access is expensive remove unnecessary
reading and writing of HOST_IRQ_STAT register.

Further, serializing access to the host data is no longer
needed and the interrupt service routine can avoid competing
on the host lock.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Suggested-by: "Jiang, Dave" <dave.jiang@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Jiang, Dave" <dave.jiang@intel.com>
Cc: linux-ide@vger.kernel.org