Fix a win7 compability issue on Asus K50IJ.

Here is the _BCM method of this laptop:
                    Method (_BCM, 1, NotSerialized)
                    {
                        If (LGreaterEqual (OSFG, OSVT))
                        {
                            If (LNotEqual (OSFG, OSW7))
                            {
                                Store (One, BCMD)
                                Store (GCBL (Arg0), Local0)
                                Subtract (0x0F, Local0, LBTN)
                                ^^^SBRG.EC0.STBR ()
                                ...
                            }
                            Else
                            {
                                DBGR (0x0B, Zero, Zero, Arg0)
                                Store (Arg0, LBTN)
                                ^^^SBRG.EC0.STBR ()
                                ...
                            }
                        }
                    }
LBTN is used to store the index of the brightness level in the _BCL.
GCBL is a method that convert the percentage value to the index value.
If _OSI(Windows 2009) is not disabled, LBTN is stored a percentage
value which is surely beyond the end of _BCL package.

http://bugzilla.kernel.org/show_bug.cgi?id=14753Signed-off-by: NZhang Rui <rui.zhang@intel.com>
Signed-off-by: NLen Brown <len.brown@intel.com>