ALSA: sc6000: Use explicit cast for __iomem pointer

The recent optimization to store an iomem pointer in card's
private_data field caused sparse warnings.  Although they are
practically harmless, add the explicit cast for avoiding the spurious
warnings.

Fixes: 9b7843d1 ("ALSA: sc6000: Assign vport directly on card's private_data")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/r/202107210511.SnFqlZMa-lkp@intel.com
Link: https://lore.kernel.org/r/20210720224902.7169-1-tiwai@suse.de


Signed-off-by: Takashi Iwai <tiwai@suse.de>