usb: dwc3: shutdown usb_phy when removing the device

We call usb_phy_init() from dwc3_core_init() during
probe, so adding usb_phy_shutdown() to dwc3_core_exit()
while removing the device so we don't keep PHYs
turned on, consuming power, unnecessarily.

Signed-off-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>