usb: gadget: function: delete an unnecessary check before rndis_add_hdr()

The rndis_add_hdr() function tests whether its argument is NULL and then
returns immediately. Thus the test around the call is not needed.

This issue was detected by using the Coccinelle software.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Felipe Balbi <balbi@ti.com>