usb: dwc3: gadget: don't wait for ep cmd IRQ

That IRQ is causing way too much trouble. We have
a different handling which was agreed with IP
provider and has been tested with FPGA and OMAP5.

Tested-by: Pratyush Anand <pratyush.anand@st.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>