EDAC/thunderx: Check debugfs file creation retval properly

edac_debugfs_create_file() returns ERR_PTR by way of the respective
debugfs function it calls, if an error occurs.

The appropriate way to verify for errors is to use IS_ERR(). Do so.

  [ bp: Rewrite all text. ]

Signed-off-by: Yeqi Fu <asuk4.q@gmail.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20230517173111.365787-1-asuk4.q@gmail.com