subsys/usb: Fix use of 'abs' in computing when to report on_idle
subsys/usb: Fix use of 'abs' in computing when to report on_idle

idle_rate is uint8_t, sof_cnt is uint32_t. The result is uint32_t, which
is the wrong type for 'abs'. Explicitly cast idle_rate to uint32_t,
subtract sof_cnt and then explicitly cast to int32_t and then use abs,
storing the result in another int32_t which matches the return type for
abs.

This quiets clang warnings about passing unsigned values to abs.

Signed-off-by: Keith Packard <fd7f967895e9f35e58ec8a62a847a54d7fa7275f@keithp.com>

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