ath10k: fix erroneous return value

The ath10k_pci_hif_exchange_bmi_msg() function may return the positive
value EIO instead of -EIO in case of error.

Signed-off-by: Anton Protopopov <a.s.protopopov@gmail.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>