serial: digicolor-usart: Don't allow CS5-6

serial: digicolor-usart: Don't allow CS5-6

Only CS7 and CS8 seem supported but CSIZE is not sanitized to CS8 in
the default: block.

Set CSIZE correctly so that userspace knows the effective value.
Incorrect CSIZE also results in miscalculation of the frame bits in
tty_get_char_size() or in its predecessor where the roughly the same
code is directly within uart_update_timeout().

Fixes: 933668170da5 (serial: driver for Conexant Digicolor USART)
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/r/20220519081808.3776-3-ilpo.jarvinen@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>