mfd: rtsx_pcr: Fix MSI enable error handling

pci_enable_msi() can return failure with both positive and negative
integers -- it returns 0 for success -- but is only tested here for
"if (ret < 0)".  This causes us to try to use MSI on the RTS5249 SD
reader in the Dell XPS 11 when enabling MSI failed, causing:

[    1.737110] rtsx_pci: probe of 0000:05:00.0 failed with error -110

Cc: stable <stable@vger.kernel.org>
Reported-by: D. Jared Dominguez <Jared_Dominguez@Dell.com>
Tested-by: D. Jared Dominguez <Jared_Dominguez@Dell.com>
Signed-off-by: Chris Ball <chris@printf.net>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/rtsx_pcr.c

@@ -1197,7 +1197,7 @@ static int rtsx_pci_probe(struct pci_dev *pcidev,
 	pcr->msi_en = msi_en;
 	if (pcr->msi_en) {
 		ret = pci_enable_msi(pcidev);
-		if (ret < 0)
+		if (ret)
 			pcr->msi_en = false;
 	}