mobile: rework writing BA to file, move to a function

mobile: rework writing BA to file, move to a function

Sometimes I am seeing error messages like this:

  DCS ERROR Failed to write BA list

The problem is that there can be several BA entries which need to
be written, and for each of them we're calling fwrite() twice.
This function returns number of items written, so the final sum
of returned values would be: len(BA list) * 2.  Thus expecting
it to be 2 regardless of len(BA list) is wrong.

Fix this by checking the sum in each iteration, not at the end.
Take a chance to refactor the code and move to a function.

Change-Id: Id8bc216c146127d9c9995379c9e56450d328f46d