staging: iio: ade7753: expanding buffer lock to cover both buffer and state protection

Dropping the extra lock (used for protecting the write frequency) by
expanding the buffer lock to cover both buffer and state protection.

Doing this by introducing a new function (__ade7753_spi_write_reg_16)
making buffer changes without locking the state, to avoid nested locks
while making device frequency changes.

Signed-off-by: Himanshi Jain <himshijain.hj@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>