drivers: serial: stm32: replace `config` parameter with `usart`

Updated function parameters from `const struct uart_stm32_config *config`
to `USART_TypeDef *usart`. This change reduces the level of pointer
indirection, which minimizes repeated dereferencing and helps reduce
the overall code size.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>