usb: musb: Remove DMA ifdef for musb_gadget.c short_packet

Let's get rid of the horrible ifdef in middle of the expression.
We can do it by adding a variable for short_packet and testing
it separately for DMA related code.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>