PCI: imx6: Fix a typo in error message

Fix a typo in the "pcie_inbound_axi clock missing or invalid" error
message.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Lucas Stach <l.stach@pengutronix.de>
CC: yurovsky@gmail.com
CC: Fabio Estevam <fabio.estevam@nxp.com>