msm: usbaudio: Add check for NULL before dereferencing

kzalloc() and usb_ifnum_to_if() both APIs can return NULL. Current
code is not checking return value and derefencing which may crash
device if it is set to NULL. Fix this by checking return value
against NULL and handling the same.

CRs-Fixed: 562273
Change-Id: I0d2c910f43321e94fc447b19ae3e3207727e24f3
Signed-off-by: Mayank Rana <mrana@codeaurora.org>