This check is hardly required and alas is wrong. 'c' might be NULL but
the chances are low that 'controller' after the container_of() becomes
NULL.

Since no other DMA implementation is doing that and musb-core does not
call it with a NULL pointer it can dropped.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Felipe Balbi <balbi@ti.com>