staging: dwc2: fix useless test for non-0

In dwc2_xfercomp_isoc_split_in(), the function has already exited
if len == 0, so no need to test it again

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>