regulator: rk8xx: Fix buck get and set enabled state on RK806

regulator: rk8xx: Fix buck get and set enabled state on RK806

Wrong POWER_EN reg is used to get and set enabled state for the RK806
buck 4 and 8 regulators, also wrong POWER_SLP_EN0 bit is used for
suspend state for the RK806 buck 1-8 regulators.

Fix this by not adding one to the zero based buck variable.

Fixes: f172575d92cd ("power: rk8xx: add support for RK806")
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>