usb: host: xhci-plat: Avoid unneeded internal declaration warning

When compiling with clang, the following warning is thrown:

drivers/usb/host/xhci-plat.c:443:36: warning: variable
'usb_xhci_acpi_match' is not needed and will not be emitted
[-Wunneeded-internal-declaration]

This happens because the reference to the usb_xhci_acpi_match table
is wrapped in the ACPI_PTR() macro which is NULL when CONFIG_ACPI
is not defined.

While __maybe_unused could be used here to quell the warning, this
patch opts to simply wrap the definition of the match table within
which follows the pattern used by other drivers.

Change-Id: I92bdee0d22a99600ff085fe7089e16b93175abbc
Signed-off-by: Jack Pham <jackp@codeaurora.org>