greybus: loopback: Drop NULL check on container_of pointer

container_of cannot return NULL and the pointer passed to this context uses
reference counter bumped inside a spinlock, so the base pointer will be
valid at this point.

Suggested-by: Johan Hovold <johan@hovoldconstulting.com>
Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>