staging: emxx_udc: remove unnecessary nullpointer checks

This patch removes nullpointer checks which are redundant. container_of
returns structure containing entity which we know for sure is not null,
so the whole structure can not be null.

Signed-off-by: Arkadiusz Lis <areklis909@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>