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>