debugfs: debugfs_create_file_size(): use IS_ERR to check for error

debugfs: debugfs_create_file_size(): use IS_ERR to check for error

debugfs_create_file() returns encoded error so use IS_ERR for checking
return value.

Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Nirmoy Das <nirmoy.das@amd.com>
Fixes: ff9fb72bc077 ("debugfs: return error values, not NULL")
Cc: stable <stable@vger.kernel.org>
References: https://gitlab.freedesktop.org/drm/amd/-/issues/1686
Link: https://lore.kernel.org/r/20210902102917.2233-1-nirmoy.das@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>