regulator: ab8500: Remove unused embedded struct expand_register

Used primarily for the AB8540 which lost support in early 2018.
It is now deemed safe to remove this legacy data structure.

Also fixes W=1 issue:

  drivers/regulator/ab8500.c:88: warning: Function parameter or member 'expand_register' not described in 'ab8500_regulator_info'

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200625163614.4001403-6-lee.jones@linaro.org
Signed-off-by: Mark Brown <broonie@kernel.org>