drivers: serial: stm32: fix non-LP UARTs with PM

Re-configure the registers if they re erased because of a STOP2 low power
mode.

Return the error code when re-initializing the UART after a STANDBY low
power mode.

Fix few typos and comments.

Run clang format on uart_stm32_pm_action()

Signed-off-by: Mickael Bosch <mickael.bosch@linux.com>