drivers: iio: imu: power off regulator only if regulator_check is enabled

The power down of asm330 regulator is not necessary if regulator_check
is disabled because regulator is initialised based on regulator_check
enablement.

Change-Id: Ida6c86258ebdc42602f3e7b858197f807d0a0ff6
Signed-off-by: Puneet Yatnal <puneet@codeaurora.org>