ahci: give another shot at clearing all bits in irq_stat

Commit ea0c62f7

 tried to clear all
bits in irq_stat but it didn't actually achieve that as irq_stat was
anded with port_map right after read.  This patch makes ahci driver
always use the unmasked value to clear irq_status.

While at it, add explanation on the peculiarities of ahci IRQ
clearing.

This was spotted by Linus Torvalds.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>