Make use of the bitfield macros thus partially hiding the complexity
of dealing with bitfields.

The patch also includes a minor fix to REG0_RATE_MASK, so far it was
set to bit 0..10, but according to the spec it's bit 0..11.

[mchehab@s-opensource.com: readd REG1_MODE_SHIFT and REG2_MODE_SHIFT
 that got removed on the original patch, as this will be used on
 another patch]
Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Reviewed-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>