serial: stm32: Use of_device_get_match_data()

This driver casts away the constness of struct stm32_usart_info that is
pointed to by the of match table. Use of_device_get_match_data() instead
of of_match_device() here and push the const throughout the code so that
we don't cast away const. This nicely avoids referencing the match table
when it is undefined with configurations where CONFIG_OF=n and fixes the
const issues.

Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: Alexandre Torgue <alexandre.torgue@st.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: <linux-serial@vger.kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210123034428.2841052-4-swboyd@chromium.org