ASoC: WL1273 FM radio: Fix breakage with MFD API changes

These changes are needed to keep up with the changes in the
MFD core and V4L2 parts of the wl1273 FM radio driver.

Use function pointers instead of exported functions for I2C IO.
Also move all preprocessor constants from the wl1273.h to
include/linux/mfd/wl1273-core.h.

Signed-off-by: Matti J. Aaltonen <matti.j.aaltonen@nokia.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>