usb: dwc2: gadget: Moved dtxfsiz backup array place

Moved dtxfsiz from dwc2_gregs_backup to dwc2_dregs_backup,
because it is device register.

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>