drivers: uart: esp32: fix poll in return value

ESP32 uart_poll_in should return 0 on success and not
the total amount of data read.

This also adds a check in fifo_fill call to
avoid negative values, otherwise it would send garbage
to uart

Closes #41352

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