If is_enabled callback is not implemented, the core assumes that the regulator
is on.

This is simpler than having a hack to retrun 1 in max8997_reg_is_enabled() if
max8997_get_enable_register() returns -EINVAL.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>