usb: gadget: s3c-hsotg: pass 'struct usb_request *' to usb_gadget_unmap_request()

'struct usb_request *' should be passed to usb_gadget_unmap_request(),
as the second argument; however, 'struct s3c_hsotg_req *' is used.

Fixed build warnings as below:

drivers/usb/gadget/s3c-hsotg.c: In function 's3c_hsotg_unmap_dma':
drivers/usb/gadget/s3c-hsotg.c:440:2: warning: passing argument 2 of 'usb_gadget_unmap_request' from incompatible pointer type
[enabled by default]
include/linux/usb/gadget.h:961:13: note: expected 'struct usb_request *' but argument is of type 'struct s3c_hsotg_req *'
drivers/usb/gadget/s3c-hsotg.c:434:22: warning: unused variable 'req' [-Wunused-variable]

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>