AP_HAL_ChibiOS: convert back to full range on IOMCU ONESHOT125 read

[IamPete1]

Fixes #13517

Simply the inverse of the calculation done here:

ardupilot/libraries/AP_HAL_ChibiOS/RCOutput.cpp

Lines 497 to 501 in 0ee8b45

	if ((iomcu_mode == MODE_PWM_ONESHOT125) && ((1U<<chan) & io_fast_channel_mask)) {
	// the iomcu only has one oneshot setting, so we need to scale by a factor
	// of 8 here for oneshot125
	io_period_us /= 8;
	}

Tested on CubeOrange