usb: dwc2: gadget: don't embed ep0 buffers

When using DMA, data of the previous setup packet can be read back
from cache because ep0 and ctrl buffers are embedded in struct s3c_hsotg.
Allocate buffers instead of embedding them.

Tested-by: Robert Baldyga <r.baldyga@samsung.com>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Mian Yousaf Kaukab <yousaf.kaukab@intel.com>
Reviewed-by: Robert Baldyga <r.baldyga@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>