usb: gadget: add a sparse endian notation

The original code works fine, but Sparse complains because it isn't
annotated properly.

drivers/usb/gadget/composite.c:793:26: warning: incorrect type in assignment (different base types)
drivers/usb/gadget/composite.c:793:26:    expected unsigned short [unsigned] [usertype] language
devel/drivers/usb/gadget/composite.c:793:26:    got restricted __le16 [usertype] <noident>
devel/drivers/usb/gadget/composite.c:795:29: warning: restricted __le16 degrades to integer
drivers/usb/gadget/composite.c:798:24: warning: incorrect type in assignment (different base types)
drivers/usb/gadget/composite.c:798:24:    expected restricted __le16 [usertype] <noident>
devel/drivers/usb/gadget/composite.c:798:24:    got unsigned short [unsigned] [usertype] language

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>