MIPS: Modify error handling

MIPS: Modify error handling

debugfs_create_file returns NULL on error so an IS_ERR test is
incorrect here and a NULL check is required.

The Coccinelle semantic patch used to make this change is as follows:
@@
expression e;
@@

  e = debugfs_create_file(...);
if(
-    IS_ERR(e)
+    !e
    )
    {
  <+...
  return
- PTR_ERR(e)
+ -ENOMEM
  ;
  ...+>
  }

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Cc: julia.lawall@lip6.fr
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/13834/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>