staging: emxx_udc: test returned value

Put NULL test on the result of the previous call instead on one of its
arguments.  A simplified version of the semantic match that finds this
problem is as follows (http://coccinelle.lip6.fr/):

// <smpl>
r@
expression *e1;
expression *e2;
identifier f;
statement S1,S2;
@@

e1 = f(...,e2,...);
(
if (e1 == NULL || ...) S1 else S2
|
*if (e2 == NULL || ...) S1 else S2
)
// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>