usb: dwc2: host: Split code out to make dwc2_do_reserve()

This no-op change splits code out of dwc2_schedule_periodic() into a
dwc2_do_reserve() function.  This makes it a little easier to follow the
logic.

Acked-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Tested-by: Heiko Stuebner <heiko@sntech.de>
Tested-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>