pci: Use a separate 'dev' variable for the PCI device

In the 'pci' command, add a separate variable to hold the PCI device. When
this code is converted to driver model, this variable will be used to hold a
struct udevice instead.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>