mfd: Staticise ab3550 register access functions

These are now exported via an ops table rather than referenced
directly and so should be staticised.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>