pci: Use common functions to read/write config

Currently we use switch() and access PCI configuration via several
functions, one for each data size. Adjust the code to use generic functions,
where the data size is a parameter.

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