ahci: compile out msi/msix infrastructure

Quoting Arnd:
    The AHCI driver is used for some on-chip devices that do not use PCI
    for probing, and it can be built even when CONFIG_PCI is disabled, but
    that now results in a build failure:

    ata/libahci.c: In function 'ahci_host_activate_multi_irqs':
    ata/libahci.c:2475:4: error: invalid use of undefined type 'struct msix_entry'
    ata/libahci.c:2475:21: error: dereferencing pointer to incomplete type 'struct msix_entry'

Add ifdef CONFIG_PCI_MSI infrastructure to compile out the multi-msi and
multi-msix code.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Tested--by: Arnd Bergmann <arnd@arndb.de>
[arnd: fix up pci enabled case]
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Fixes: d684a90d ("ahci: per-port msix support")
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Tejun Heo <tj@kernel.org>