usb: dwc3: gadget: split special cases of ep_queue()

It makes it easier to read and also avoids
setting DWC3_EP_PENDING_REQUEST just so the
next branch evaluates true.

No functional changes otherwise.

Cc: Pratyush Anand <pratyush.anand@st.com>
Cc: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>