ahci: Move interrupt enablement code to a separate function

This patch refactors ahci_init_interrupts() and moves msi code to a
separate function. Need the split since we add msix initialization in
a later patch. The initialization for msix will be done after msi but
before intx.

Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: Tejun Heo <tj@kernel.org>