usb: gadget: udc: net2280: Move ASSERT_OUT_NAKING into out_flush

ASSERT_OUT_NAKING was only called by out_flush and was hidden behind a
ifdef.

This patch moves the inline function into out_flush and remove the
ifdef. The user can decide to print the debug message or not via dynamic
printk

Signed-off-by: Ricardo Ribalda Delgado <ricardo.ribalda@gmail.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>