x86/resctrl: Cleanup cbm_ensure_valid()

A recent fix to the cbm_ensure_valid() function left some coding style
issues that are now addressed:

- Return a value instead of using a function parameter as input and
  output
- Use if (!val) instead of if (val == 0)
- Follow reverse fir tree ordering of variable declarations

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fenghua.yu@intel.com
Cc: tony.luck@intel.com
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/15ba03856f1d944468ee6f44e3fd7aa548293ede.1561408280.git.reinette.chatre@intel.com