usb: dwc3: gadget: fix writing NYET threshold

usb: dwc3: gadget: fix writing NYET threshold

Before writing a new value to the register, the old value needs to be
masked out for the new value to be programmed as intended, because at
least in some cases the reset value of that field is 0xf (max value).

At the moment, the dwc3 core initialises the threshold to the maximum
value (0xf), with the option to override it via a DT. No upstream DTs
seem to override it, therefore this commit doesn't change behaviour for
any upstream platform. Nevertheless, the code should be fixed to have
the desired outcome.

Do so.

Fixes: 80caf7d21adc ("usb: dwc3: add lpm erratum support")
Cc: stable@vger.kernel.org # 5.10+ (needs adjustment for 5.4)
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Acked-by: Thinh Nguyen <Thinh.Nguyen@synopsys.com>
Link: https://lore.kernel.org/r/20241209-dwc3-nyet-fix-v2-1-02755683345b@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>