media: usb: Check az6007_read() return value

If az6007_read() returns error, there is no sence to continue.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 3af2f4f1 ("[media] az6007: Change the az6007 read/write routine parameter")
Signed-off-by: Daniil Dulov <d.dulov@aladdin.ru>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>