tty: serdev-ttyport: return actual baudrate from ttyport_set_baudrate

Instead of returning the requested baudrate, we better return the
actual one because it isn't always the same.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>