usb-audio: work around wrong frequency in CM6501 descriptors

The C-Media CM6501 chip's descriptors say that altsetting 5 supports
48 kHz, but it actually plays at 96 kHz.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>