usb: gadget: Use ERR_CAST inlined function instead of ERR_PTR(PTR_ERR(...))

trivial patch converting ERR_PTR(PTR_ERR()) into ERR_CAST().
No functional changes.

Signed-off-by: Duan Jiong <duanj.fnst@cn.fujitsu.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>