serial: stm32: Fix unused-variable warning

serial: stm32: Fix unused-variable warning

If CONFIG_SERIAL_EARLYCON and CONFIG_OF are both not set,
gcc warns about unused variable:

drivers/tty/serial/stm32-usart.c:83:32: error: ‘stm32h7_info’ defined but not used [-Werror=unused-variable]
 static struct stm32_usart_info stm32h7_info = {
                                ^~~~~~~~~~~~
drivers/tty/serial/stm32-usart.c:61:32: error: ‘stm32f7_info’ defined but not used [-Werror=unused-variable]
 static struct stm32_usart_info stm32f7_info = {
                                ^~~~~~~~~~~~
drivers/tty/serial/stm32-usart.c:40:32: error: ‘stm32f4_info’ defined but not used [-Werror=unused-variable]
 static struct stm32_usart_info stm32f4_info = {
                                ^~~~~~~~~~~~
cc1: all warnings being treated as errors

Mark these variables as __maybe_unused  to fix this.

Fixes: c7039ce904c0 ("serial: stm32: make info structs static to avoid sparse warnings")
Signed-off-by: Ren Zhijie <renzhijie2@huawei.com>
Link: https://lore.kernel.org/r/20220926025826.44145-1-renzhijie2@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>