ath9k: Fix error check in ath9k_hw_read_revisions() for PCI devices

When the error check in ath9k_hw_read_revisions() was added, it checked for
-EIO which is what ath9k_regread() in the ath9k_htc driver uses. However,
for plain ath9k, the register read function uses ioread32(), which just
returns -1 on error. So if such a read fails, it still gets passed through
and ends up as a weird mac revision in the log output.

Fix this by changing ath9k_regread() to return -1 on error like ioread32()
does, and fix the error check to look for that instead of -EIO.

Fixes: 2f90c7e5d0 ("ath9k: Check for errors when reading SREV register")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210326180819.142480-1-toke@redhat.com

drivers/net/wireless/ath/ath9k/htc_drv_init.c

@@ -246,7 +246,7 @@ static unsigned int ath9k_regread(void *hw_priv, u32 reg_offset)
 	if (unlikely(r)) {
 		ath_dbg(common, WMI, "REGISTER READ FAILED: (0x%04x, %d)\n",
 			reg_offset, r);
-		return -EIO;
+		return -1;
 	}
 
 	return be32_to_cpu(val);

drivers/net/wireless/ath/ath9k/hw.c

@@ -286,7 +286,7 @@ static bool ath9k_hw_read_revisions(struct ath_hw *ah)
 
 	srev = REG_READ(ah, AR_SREV);
 
-	if (srev == -EIO) {
+	if (srev == -1) {
 		ath_err(ath9k_hw_common(ah),
 			"Failed to read SREV register");
 		return false;