thinkpad_acpi: Fix build error when CONFIG_SND_MAX_CARDS > 32

SNDRV_CARDS can be specified via Kconfig since 3.11 kernel, so this
can be over 32bit integer range, which leads to a build error.

Cc: <stable@vger.kernel.org> [v3.11+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Matthew Garrett <matthew.garrett@nebula.com>