sound: usb: Add NULL check on return value of iommu_domain_alloc()

Upon allocation failure iommu_domain_alloc() returns NULL.
Driver is not checking for NULL value, which leads to NULL
pointer dereference later.

Change-Id: I9788412af3a34aca8fa8519e7ae6f3349f263a1e
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>