ALSA: mts64: fix NULL pointer dereference

While registering pardev, the irq_func was also registered. As a
result when we tried to probe for the card, an interrupt was generated
and in the ISR we tried to dereference private_data. But private_data
is still NULL as we have not yet done snd_mts64_create(). Lets probe
for the card after mts64 is created.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Fixes: 94a57350 ("ALSA: mts64: use new parport device model")
Signed-off-by: Sudip Mukherjee <sudip.mukherjee@codethink.co.uk>
Signed-off-by: Takashi Iwai <tiwai@suse.de>