serial: stm32: clean probe and remove port deinit

Clean probe and remove port deinit by moving clk_disable_unprepare in a
new dedicated deinit_port function.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Link: https://lore.kernel.org/r/20210106162203.28854-8-erwan.leray@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>