PCI/VGA: Simplify vga_client_register()

Reorganize vga_client_register() to avoid the goto and the need to save the
return value.  Update the kernel-doc to reflect -ENODEV on failure.  No
functional change intended.

[bhelgaas: drop "ret" variable, commit log]
Link: https://lore.kernel.org/r/20230808223412.1743176-8-sui.jingfeng@linux.dev


Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>