serial: mvebu-uart: clarify the baud rate derivation

The current comment in ->set_baud_rate() is rather incomplete as it
fails to describe what are the actual stages for the baudrate
derivation. Replace this comment with something more explicit and
close to the functional specification. Also adapt the variable names
to it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>