PCI: aardvark: Fix support for bus mastering and PCI_COMMAND on emulated bridge

From very vague, ambiguous and incomplete information from Marvell we
deduced that the 32-bit Aardvark register at address 0x4
(PCIE_CORE_CMD_STATUS_REG), which is not documented for Root Complex mode
in the Functional Specification (only for Endpoint mode), controls two
16-bit PCIe registers: Command Register and Status Registers of PCIe Root
Port.

This means that bit 2 controls bus mastering and forwarding of memory and
I/O requests in the upstream direction. According to PCI specifications
bits [0:2] of Command Register, this should be by default disabled on
reset. So explicitly disable these bits at early setup of the Aardvark
driver.

Remove code which unconditionally enables all 3 bits and let kernel code
(via pci_set_master() function) to handle bus mastering of Root PCIe
Bridge via emulated PCI_COMMAND on emulated bridge.

Link: https://lore.kernel.org/r/[email protected]
Fixes: 8a3ebd8de328 ("PCI: aardvark: Implement emulated root PCI bridge config space")
Signed-off-by: Pali Rohár <[email protected]>
Signed-off-by: Marek Behún <[email protected]>
Signed-off-by: Lorenzo Pieralisi <[email protected]>
Cc: [email protected] # b2a56469d550 ("PCI: aardvark: Add FIXME comment for PCIE_CORE_CMD_STATUS_REG access")

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