libahci: Refactoring of ahci_single_irq_intr function.

This patch refactors the ahci_single_irq_intr function and also rename
it to ahci_single_level_irq_intr as it handles a level triggered latch.

Signed-off-by: Suman tripathi <stripathi@apm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>