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.