kselftest: alsa: fixed a print formatting warning

A statement used %d print formatter where %s should have
been used. The same has been fixed in this commit.

Signed-off-by: Ghanshyam Agrawal <ghanshyam1898@gmail.com>
Link: 5aaf9eff ("kselftest: alsa: Add simplistic test for ALSA mixer controls kselftest")
Link: https://lore.kernel.org/r/20231217080019.1063476-1-ghanshyam1898@gmail.com


Signed-off-by: Takashi Iwai <tiwai@suse.de>