regulator: tps6524x: Convert fixed ilimsel to table based

This patch refactors the code to get rid of the fixed_ilimsel and FIXED_ILIMSEL
flag usage, and convert all the fixed ilimsel to table based (with one entry in
the table).

We can differentiate fixed ilimsel by checking info->n_ilimsels == 1,
thus FIXED_ILIMSEL flag can be removed.

This change makes the logic of the code simpler as all the ilimsels are table
based now.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>