regulator: Fix the logic of tps65910_get_mode

We actually clear LDO_ST_ON_BIT for standby mode in tps65910_set_mode.
Fix the logic in tps65910_get_mode.

Supply state (EEPROM bits):
ST[1:0] = 00 : Off
ST[1:0] = 01 : On high power (ACTIVE)
ST[1:0] = 10 : Off
ST[1:0] = 11 : On low power (SLEEP)

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>