uart: unused warning

[fpistm]

Since #2701, if UART does not support the rx, tx and data invert warnings are raised about unused variables:
Example with STM32F4 target:

.arduino15/packages/STMicroelectronics/hardware/stm32/2.10.1/libraries/SrcWrapper/src/stm32/uart.c: In function 'uart_init':
.arduino15/packages/STMicroelectronics/hardware/stm32/2.10.1/libraries/SrcWrapper/src/stm32/uart.c:118:110: warning: unused parameter 'rx_invert' [-Wunused-parameter]
  118 | bool uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t parity, uint32_t stopbits, bool rx_invert, bool tx_invert, bool data_invert)
      |                                                                                                              ^
.arduino15/packages/STMicroelectronics/hardware/stm32/2.10.1/libraries/SrcWrapper/src/stm32/uart.c:118:126: warning: unused parameter 'tx_invert' [-Wunused-parameter]
  118 | bool uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t parity, uint32_t stopbits, bool rx_invert, bool tx_invert, bool data_invert)
      |                                                                                                                              ^
.arduino15/packages/STMicroelectronics/hardware/stm32/2.10.1/libraries/SrcWrapper/src/stm32/uart.c:118:142: warning: unused parameter 'data_invert' [-Wunused-parameter]
  118 | bool uart_init(serial_t *obj, uint32_t baudrate, uint32_t databits, uint32_t parity, uint32_t stopbits, bool rx_invert, bool tx_invert, bool data_invert)
      |