ALSA: firewire-motu: fix null pointer dereference when polling hwdep character device

ALSA: firewire-motu: fix null pointer dereference when polling hwdep character device

ALSA firewire-motu driver recently got support for event notification via
ALSA HwDep interface for register DSP models. However, when polling ALSA
HwDep cdev, the driver can cause null pointer dereference for the other
models due to accessing to unallocated memory or uninitialized memory.

This commit fixes the bug by check the type of model before accessing to
the memory.

Reported-by: kernel test robot <lkp@intel.com>
Suggested-by: Takashi Iwai <tiwai@suse.de>
Fixes: 634ec0b2906e ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211027125529.54295-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>