usb: dwc3: st: Use explicit reset_control_get_exclusive() API

We're making all reset line users specify whether their lines are
shared with other IP or they operate them exclusively.  In this case
the line is exclusively used only by this IP, so use the *_exclusive()
API accordingly.

Acked-by: Felipe Balbi <felipe.balbi@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>