pci: xilinx: Fix doc comments on config space accessors

pci: xilinx: Fix doc comments on config space accessors

These take the 'struct udevice *' as an argument, not the
'struct xilinx_pcie *` which is a local variable. Fix the comments to
match the code.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>