NOR/CFI: simplify bufferwsize computation

NOR/CFI: simplify bufferwsize computation

Review and simplify computation of bufferwsize.
Add comments about variables' meaning.

The same code is present 3 times in the file.
Current patch updates all the 3 instances.

Step 1)
  Replace "switch(bank->chip_width) {...}".
  Illegal values of bank->chip_width are already dropped.
  For legal values, the code is equivalent to:
        bufferwsize = buffersize / bank->chip_width;

Step 2)
  The above code replacement plus the following line:
        bufferwsize /= (bank->bus_width / bank->chip_width);
  is merged in a single formula:
        bufferwsize = (buffersize / bank->chip_width) /
                (bank->bus_width / bank->chip_width);
  and simplified as:
        bufferwsize = buffersize / bank->bus_width;

Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>