The desc_id variable should not be a static variable.

The rest of the code assumes the desc_id must less than TPSxxxxx_NUM_REGULATOR.
If we set desc_id to be a static variable, checking the return value of
rdev_get_id() may return error.
Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Anuj Aggarwal <anuj.aggarwal@ti.com>
Cc: Graeme Gregory <gg@slimlogic.co.uk>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>