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

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

[ Upstream commit 74f7957c9b1b95553faaf146a2553e023a9d1720 ]

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

Fixes: e3037485c68e ("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
Signed-off-by: Sasha Levin <sashal@kernel.org>