drivers: usb: device: stm32: Use macros for frequencies
drivers: usb: device: stm32: Use macros for frequencies

Rather than explicitly setting out the frequnecy in full, use the MHZ()
macro.

Signed-off-by: Chris Collins <711c73f64afdce07b7e38039a96d2224209e9a6c@realsimgear.com>

galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr