mfd: Move AB3100 to __devinit

Since there is no discardable probe() function in the I2C device
framework, let's just tag it __devinit and take the footprint hit
rather than seeing the compilation warnings every day.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>