iio: imu: inv_mpu6050: fix user_ctrl register overwritten

When in spi mode, we are setting i2c disable bit in user_ctrl
register. But the register is overwritten after when turning fifo
on. So save user_ctrl init value and always use it when updating
the register.

Signed-off-by: Jean-Baptiste Maneyrol <jmaneyrol@invensense.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>