gnss: ubx: use new helper to remove open coded regulator handling

v_bckp shall always be enabled as long as the device exists. We now have
a regulator helper for that, use it.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Johan Hovold <johan@kernel.org>