selftests/alsa: Fix memory leak in find_controls error path

selftests/alsa: Fix memory leak in find_controls error path

In find_controls(), card_data is allocated with malloc() but when
snd_ctl_open_lconf() fails, the code jumps to next_card without
freeing the allocated memory. This results in a memory leak for
each card where snd_ctl_open_lconf() fails.

Add free(card_data) before goto next_card to ensure proper cleanup
of the allocated memory in the error path.

Fixes: 5aaf9efffc57 ("kselftest: alsa: Add simplistic test for ALSA mixer controls kselftest")
Signed-off-by: Malaya Kumar Rout <malayarout91@gmail.com>
Link: https://patch.msgid.link/20260704105736.94874-1-malayarout91@gmail.com
Signed-off-by: Takashi Iwai <tiwai@suse.de>