AHCI: Cleanup checking of multiple MSIs/SLM modes

Sharing Last Message (SLM) mode is currently checked in two
functions: ahci_host_activate() and ahci_init_interrupts().
This update consolidates SLM mode check with activation of
multiple MSIs mode.

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: linux-ide@vger.kernel.org