drivers: serial: simplify STM32 UART clock initialization

STM32 UART driver uses a macro for clock initialization, that
is difficult to read and incompatible with needed changes to
fix STM32F0 series UART problems.
This change switches to using the full clock bus names in UART
init functions removing the macro-magic and increasing
readability.

Signed-off-by: Daniel Wagenknecht <wagenknecht@clage.de>