Fix capacity report error in Huawei driver

Fix capacity report error in Huawei driver

Currently the capacity report is incorrect due
to wrong way code clear.

The previously right way is:
result = var / 1024.0 / 1024.0 / 2

But currently we use a constant CAPACITY_UNIT to make the code cleaner
like this:
CAPACITY_UNIT = 1024.0 / 1024.0 / 2
result = var / CAPACITY_UNIT
We want the CAPACITY_UNIT work like a *real* constant like C
language, but in python, it is not a *real* constant, it's
just a variable. It will calculate the CAPACITY_UNIT first,
so the finally result is:
result = var / 0.5
And this is wrong.

This patch will fix this.

Closes-Bug: #1498452
Change-Id: I4dc20ef8805d7c414fedb1dc441deb3f9fcd75b5