x86/platform/iosf_mbi: Check return value of debugfs_create properly

x86/platform/iosf_mbi: Check return value of debugfs_create properly

The code checks the result of the first debugfs_create call several
times and fails to check the result of the subsequent calls due to
missing assigments.

Add the missing assignments and check only for !res because
debugfs_create() returns only NULL on error and not an encoded error
code.

[ tglx: Massaged changelog ]

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: David E . Box <david.e.box@linux.intel.com>
Link: http://lkml.kernel.org/r/1436366709-17683-3-git-send-email-andriy.shevchenko@linux.intel.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>