usb: dwc2: gadget: move phy control calls out of pullup() method

This patch moves phy enable/disable calls from pullup() method to
udc_start/stop functions. This solves the issue related to limited caller
context for PHY functions, because they cannot be called from non-sleeping
context. This is also a preparation for using soft-disconnect feature of
udc controller in pullup() method.

Acked-by: Paul Zimmerman <paulz@synopsys.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>