usb: gadget: mv_u3d: fix unused-but-set-variable warnings

This patch fixes the following (W=1) warnings:

drivers/usb/gadget/udc/mv_u3d_core.c: In function ‘mv_u3d_process_ep_req’:
drivers/usb/gadget/udc/mv_u3d_core.c:124:6: warning: variable ‘trb_complete’ set but not used [-Wunused-but-set-variable]
  int trb_complete, actual, remaining_length = 0;
      ^
drivers/usb/gadget/udc/mv_u3d_core.c:123:28: warning: variable ‘curr_ep_context’ set but not used [-Wunused-but-set-variable]
  struct mv_u3d_ep_context *curr_ep_context;
                            ^
drivers/usb/gadget/udc/mv_u3d_core.c:122:13: warning: variable ‘cur_deq_lo’ set but not used [-Wunused-but-set-variable]
  dma_addr_t cur_deq_lo;
             ^
drivers/usb/gadget/udc/mv_u3d_core.c: In function ‘mv_u3d_ep_enable’:
drivers/usb/gadget/udc/mv_u3d_core.c:530:28: warning: variable ‘ep_context’ set but not used [-Wunused-but-set-variable]
  struct mv_u3d_ep_context *ep_context;
                            ^
drivers/usb/gadget/udc/mv_u3d_core.c: In function ‘mv_u3d_ep_disable’:
drivers/usb/gadget/udc/mv_u3d_core.c:636:28: warning: variable ‘ep_context’ set but not used [-Wunused-but-set-variable]
  struct mv_u3d_ep_context *ep_context;
                            ^
In doing so, it removes calls to ioread32 function which does I/O with
the device, but I hope the reads don’t have any side effects that are
needed.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>