usb: dwc2: gadget: don't overwrite DCTL register on NAKEFF interrupts

When receiving GINTSTS_GINNAKEFF or GINTSTS_GOUTNAKEFF interrupt,
DCTL will be overwritten with DCTL_CGOUTNAK or DCTL_CGNPINNAK values.
Instead of overwriting it, write only needed bits.

It could cause an issue if GINTSTS_GINNAKEFF or GINTSTS_GOUTNAKEFF
interrupt is received after dwc2 disabled pullup by writing
DCTL_SFTDISCON bit.
Pullup will then be re-enabled whereas it should not.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Gregory Herrero <gregory.herrero@intel.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>