drivers/clock_control: stm32u5: Wrong bus check in _get_rate()

Checking that provided pclken->bus fits bus range in
clock_control_get_rate() is wrong as it could actually be a source
clock and hence not belong to this range.
Also, this kind of check is just not needed since this function should
not be used before call to clock_control_on() or clock_control_configure()
which do the required verification.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>