ALSA: ctxfi: ctatc: added reference to snd_card

added reference of the card in the convert_format function
so that we can know which card has called the function.
this reference of the snd_card will actually be used in a later patch
to convert the pr_* macro to dev_*.

Signed-off-by: Sudip Mukherjee <sudip@vectorindia.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>