ahci: Ensure "MSI Revert to Single Message" mode is not enforced

The AHCI specification allows hardware to choose to revert to
single MSI mode when fewer messages are allocated than requested.
Yet, at least ICH10 chipset reverts to single MSI mode even when
enough messages are allocated in some cases (see below).

This update forces the driver to not rely on initialization of
multiple MSIs mode alone and always check if "MSI Revert to
Single Message" (MRSM) mode was enforced by the controller and
fallback to the single MSI mode in case it did.

That prevents a situation when the driver configured multiple
per-port IRQ handlers, but the controller sends all port's
interrupts to a single IRQ, which could easily screw up the
interrupt handling and lead to delays and possibly crashes.

The fix was tested on a 6-port controller that successfully
reverted to the single MSI mode:

00:1f.2 SATA controller: Intel Corporation 82801JI (ICH10 Family) SATA
AHCI Controller (prog-if 01 [AHCI 1.0])
	Subsystem: Super Micro Computer Inc Device 10a7
	Flags: bus master, 66MHz, medium devsel, latency 0, IRQ 101
	I/O ports at f110 [size=8]
	I/O ports at f100 [size=4]
	I/O ports at f0f0 [size=8]
	I/O ports at f0e0 [size=4]
	I/O ports at f020 [size=32]
	Memory at fbf00000 (32-bit, non-prefetchable) [size=2K]
	Capabilities: [80] MSI: Enable+ Count=1/16 Maskable- 64bit-
	Capabilities: [70] Power Management version 3
	Capabilities: [a8] SATA HBA v1.0
	Capabilities: [b0] PCI Advanced Features
	Kernel driver in use: ahci

With 6 ports just 8 MSI vectors should be enough, but the adapter
enforces the MRSM mode when less than 16 vectors are written to
the Multiple Messages Enable PCI register. I instigated MRSM mode
by forcing @nvec to 8 in ahci_init_interrupts().

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