tty: serial: stm32-usart: Remove set but unused 'cookie' variables

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serial/stm32-usart.c: In function ‘stm32_transmit_chars_dma’:
 drivers/tty/serial/stm32-usart.c:353:15: warning: variable ‘cookie’ set but not used [-Wunused-but-set-variable]
 drivers/tty/serial/stm32-usart.c: In function ‘stm32_of_dma_rx_probe’:
 drivers/tty/serial/stm32-usart.c:1090:15: warning: variable ‘cookie’ set but not used [-Wunused-but-set-variable]

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Gerald Baeza <gerald.baeza@st.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-stm32@st-md-mailman.stormreply.com
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-29-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>