usb: gadget: Potential NULL dereference on allocation error

The source_sink_alloc_func() function is supposed to return error
pointers on error.  The function is called from usb_get_function() which
doesn't check for NULL returns so it would result in an Oops.

Of course, in the current kernel, small allocations always succeed so
this doesn't affect runtime.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>