serial: stm32: change register's offset type from u8 to u16

USART has registers above 0xff offset, so extend variable type to u16.
And change UNDEF_REG to 0xffff.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20240112095300.2004878-4-valentin.caron@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>