drm/i2c: tda998x: set the PLL division factor in range 0..3

The predivider division factor of the register PLL_SERIAL_2 is in the
range 0..3, the value 0 being used for a division by 1.

Tested-by: Russell King <rmk+kernel@arm.linux.org.uk>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Jean-Francois Moine <moinejf@free.fr>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>