PCI: pciehp: Reduce PCIe slot_ctrl to 16 bits

4283c70e ("PCI: pciehp: Make pcie_wait_cmd() self-contained") added
a cache of the most recent command written to the Slot Control register.
This register is only 16 bits wide, but the cache ("slot_ctrl") is 32 bits.

Reduce slot_ctrl to a u16 so it matches the register size.  No functional
change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>