usb: musb: dsps: fix devm_ioremap_resource error detection code

devm_ioremap_resource returns an ERR_PTR value, not NULL, on failure.
Furthermore, the value returned by devm_ioremap_resource should be tested.

A simplified version of the semantic match that finds this problem is as
follows: (http://coccinelle.lip6.fr/

)

// <smpl>
@@
expression e,e1;
statement S;
@@

*e = devm_ioremap_resource(...);
if (!e1) S

// </smpl>

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Felipe Balbi <balbi@ti.com>