sb/intel/lynxpoint: Handle H81 only having 6 PCIe root ports

The H81 chipset is the only non-LP Lynx Point chipset with 6 PCIe root
ports, all others have 8 [1]. The existing PCIe code assumed that all
non-LP chipsets had 8 root ports, which meant that port 6 would not be
considered the last root port on H81, so `root_port_commit_config()`
would not run. Ultimately, while PCIe still worked on H81, all the root
ports would remain enabled, even if disabled in the devicetree.

Also, remove `PCI_DEVICE_ID_INTEL_LYNXPOINT_MOB_DESK_{MIN,MAX}`, as they
are unused, and the MAX constant is incorrect.

Interestingly, this fixes an issue where GRUB is unable to halt the
system.

Tested on an ASRock H81M-HDS. The root ports disabled in the devicetree
do indeed end up disabled.

[1] Intel® 8 Series/C220 Series Chipset Family Platform Controller Hub
    (PCH) Datasheet, revision 003, document number 328904.

Change-Id: If3ce217e8a4f4ea4e111e4525b03dbbfc63f92b0
Signed-off-by: Tristan Corrick <tristan@corrick.kiwi>
Reviewed-on: https://review.coreboot.org/c/30077
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Patrick Rudolph <siro@das-labor.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asrock/h81m-hds/devicetree.cb

@@ -102,8 +102,6 @@ chip northbridge/intel/haswell
 			device pci 1c.5 on      # PCIe 1x slot
 				subsystemid 0x1849 0x8c1a
 			end
-			device pci 1c.6 off end # PCIe port #7
-			device pci 1c.7 off end # PCIe port #8
 			device pci 1d.0 on      # EHCI controller #1
 				subsystemid 0x1849 0x8c26
 			end