usb: dwc3: gadget: CSP is only valid for OUT endpoints

CSP bit is only valid for OUT endpoints. Synopsys
databook is unclear if HW ignores CSP for IN
endpoints (chances are, it does) but to avoid
problems, let's make sure to set CSP only when valid
to do so.

Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>