ALSA: usb-audio: Fix invalid use of sizeof in parse_uac_endpoint_attributes()

sizeof() when applied to a pointer typed expression gives the
size of the pointer, not that of the pointed data.

Fixes: 7edf3b5e ("ALSA: usb-audio: AudioStreaming Power Domain parsing")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>