uart: esp32: move hal instance to data level

After #41918, DEV_CFG removal triggered discard qualifer warning
during build. As uart_hal functions don't required const qualifier
and uart_context_hal_t has modified data, this PR moves hal instace
to data struct instead of config struct.

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