USB: usbaudio: handle kcalloc failure

sound/usb/usbaudio.c (check_hw_params_convention): Handle kcalloc failure.

Signed-off-by: Jim Meyering <meyering@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>