PCI: pciehp: Rename controller struct members for clarity

Of the members which were just moved from pciehp's slot struct to the
controller struct, rename "lock" to "state_lock" and rename "work" to
"button_work" for clarity.  Perform the rename separately to the
unification of the two structs per Sinan's request.

No functional change intended.

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Cc: Sinan Kaya <okaya@kernel.org>