wifi: rtw88: debug: Fix the NULL vs IS_ERR() bug for debugfs_create_file()

Since debugfs_create_file() return ERR_PTR and never return NULL, so use
IS_ERR() to check it instead of checking NULL.

Fixes: e3037485 ("rtw88: new Realtek 802.11ac driver")
Signed-off-by: Jinjie Ruan <ruanjinjie@huawei.com>
Acked-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230919050651.962694-1-ruanjinjie@huawei.com