driver: serial: Move STM32 UART driver to LL API

STM32 UART driver was using a mix of LL/HAL APIs.  This commit removes
calls to HAL API and replaces them with LL APIs.  No functional change
have been seen during non regression testing.

But we could note a direct gain of 1Kb of ROM

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