fix detection of unused slots (N.A.) because of trailing spaces

areca plugin doesn't recognize unused slots (N.A.) correctly on our server. the attached patch fixes the problem for me by removing trailing spaces on the usage variable before checking for N.A.

Areca CLI, Version: 1.12.0, Arclib: 342, Date: Mar 18 2013( Linux )

```
  # Ch# ModelName                       Capacity  Usage
===============================================================================
  1  1  WDC WD5000YS-01MPB1              500.1GB  Raid Set # 00
  2  2  WDC WD5002ABYS-02B1B0            500.1GB  Raid Set # 00
  3  3  WDC WD5000YS-01MPB1              500.1GB  Raid Set # 00
  4  4  WDC WD5003ABYX-01WERA1           500.1GB  Raid Set # 00
  5  5  WDC WD5000ABYS-01TNA0            500.1GB  Raid Set # 00
  6  6  WDC WD5003ABYX-01WERA0           500.1GB  Raid Set # 00
  7  7  WDC WD5000YS-01MPB1              500.1GB  Raid Set # 00
  8  8  WDC WD5000YS-01MPB1              500.1GB  Raid Set # 00
  9  9  WDC WD5000YS-01MPB1              500.1GB  Raid Set # 00
 10 10  N.A.                               0.0GB  N.A.
 11 11  N.A.                               0.0GB  N.A.
 12 12  N.A.                               0.0GB  N.A.
 13 13  N.A.                               0.0GB  N.A.
 14 14  N.A.                               0.0GB  N.A.
 15 15  N.A.                               0.0GB  N.A.
 16 16  N.A.                               0.0GB  N.A.
===============================================================================