staging: dwc2: add helper variable to simplify code

Now a register is masked only in once place, instead of twice. This
makes the two uses of this value shorter so they no longer need to be
linewrapped.

Signed-off-by: Matthijs Kooijman <matthijs@stdin.nl>
Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>