drivers: serial: uart_stm32: tx/rx swap in init()

Previously, the uart_stm32 driver was extended in #44487 to support
swapping the tx and rx pins of supported STM32 UART peripherals.
However, the original change applied this configuration during the
call to the uart_stm32_configure() function. This has now been added to
the uart_stm32_init() function to ensure this behavior on startup for
ports like the virtual com port.

Signed-off-by: Peter Maxwell Warasila <madmaxwell@soundcomesout.com>