usb: musb: gadget: remove remaining DMA ifdeferry

Commit fb91cddc ("usb: musb: Remove DMA
ifdef for musb_gadget.c short_packet") tried
to remove DMA ifdeferry from musb_gadget.c
but ended up leaving some around.

Remove them so that when building kernels with
all DMA engines enabled, we don't end up trying
to allocte channels twice.

Signed-off-by: Felipe Balbi <balbi@ti.com>