regulator: fixed: return get/list_voltage()

The commit c368e5fc2a190923b786f2de3e79430ea3566a25 removed the get and
list functions from the fixed regulator. Unfortunately, these
functions are used by the MMC core to set the OCR mask, causing some
cards to break. Namely, this breaks the TI wilink wl18xx WiFi card on
OMAP platforms.

Revert the change and use the core descriptor to return the voltage.

Signed-off-by: Arik Nemtsov <arik@wizery.com>