regulator: ab8500: Fix build error

Fix below build error:

  CC      drivers/regulator/ab8500.o
drivers/regulator/ab8500.c:500:23: error: 'ab8500_regulator_fixed_ops' undeclared here (not in a function)
drivers/regulator/ab8500.c: In function 'ab8500_regulator_probe':
drivers/regulator/ab8500.c:972:8: warning: assignment from incompatible pointer type [enabled by default]
make[2]: *** [drivers/regulator/ab8500.o] Error 1
make[1]: *** [drivers/regulator] Error 2
make: *** [drivers] Error 2

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>