ALSA: hda/ca0132: Fix using plain integer as NULL pointer in add_tuning_control

ALSA: hda/ca0132: Fix using plain integer as NULL pointer in add_tuning_control

The 'add_tuning_control' function initializes two pointers using the
integer 0 instead of the NULL pointer, triggering sparse warnings.
Replaced both instaces of '0' with 'NULL' to resolve the type mismatch
and suppress the sparse warnings.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202506100842.d8lwwdwU-lkp@intel.com/
Signed-off-by: Lucy Thrun <lucy.thrun@digital-rabbithole.de>
Link: https://patch.msgid.link/20250610175012.918-2-lucy.thrun@digital-rabbithole.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>