wifi: ath11k: fix return value check in ath11k_ahb_probe()

ioremap() returns NULL pointer not PTR_ERR() when it fails,
so replace the IS_ERR() check with NULL pointer check.

Fixes: b42b3678 ("wifi: ath11k: remap ce register space for IPQ5018")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230217030031.4021289-1-yangyingliang@huawei.com