power: axp221: Remove switch case to simplify axp_set_eldo

The ELDO enable bits and registers are contiguous for axp221. Instead
of a switch case testing against the index, just use the index to shift
the bit or register offset.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>