ath10k: fix a NULL vs IS_ERR() check

The devm_memremap() function doesn't return NULLs, it returns error
pointers.

Fixes: ba94c753

 ("ath10k: add QMI message handshake for wcn3990 client")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>