drivers: uart: esp32: apply correct mask for TX/RX signal inversion

Build a combined mask from the tx_invert and rx_invert flags and pass it
to uart_hal_inverse_signal(). Only invoke the HAL call when the mask is
non-zero, preventing unintended inversions and eliminating redundant
calls when no inversion is requested.

Signed-off-by: Sylvio Alves <sylvio.alves@espressif.com>