iio: pressure: mprls0025pa: introduce tx buffer

iio: pressure: mprls0025pa: introduce tx buffer

Use a tx_buf that is part of the priv struct for transferring data to
the sensor instead of relying on a devm_kzalloc()-ed array.
Remove the .init operation in the process.

Reviewed-by: Marcelo Schmitt <marcelo.schmitt1@gmail.com>
Signed-off-by: Petre Rodan <petre.rodan@subdimension.ro>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>