PCI: shpchp: Remove unused shpc_writeb()

Fix the following clang warning:

  drivers/pci/hotplug/shpchp_hpc.c:177:20: warning: unused function 'shpc_writeb' [-Wunused-function].

Link: https://lore.kernel.org/r/1618475422-96531-1-git-send-email-jiapeng.chong@linux.alibaba.com


Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Krzysztof Wilczyński <kw@linux.com>