PCI: designware: Check private_data validity in single place

The driver had checks for this sprinkled all over.  As we call
sys_to_pcie() before every instance of this check, we can move the
check to this single location to make things clear.

Removing the statements after BUG[_ON]() is safe as the kernel is halted at
this point anyway.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Acked-by: Mohit Kumar <mohit.kumar@st.com>