tty/serial: atmel: fix out of range clock divider handling

Use MCK_DIV8 when the clock divider is > 65535. Unfortunately the mode
register was already written thus the clock selection is ignored.

Fix by writing the mode register after calculating the baudrate.

Signed-off-by: David Engraf <david.engraf@sysgo.com>
Link: https://lore.kernel.org/r/20191211162954.8393-1-david.engraf@sysgo.com


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