MIPS: pci-rt2880: remove unneeded locks

Mirror pci-rt3883 fix from commit e5067c71 ("MIPS: pci-rt3883:
Remove odd locking in PCI config space access code"). pci-rt2880 shares
the driver layout with pci-rt3883 and the same reasons apply.

Caller (generic PCI code) already does proper locking, so no need to add
another one here. Local PCI read/write functions are never called
simultaneously, also they do not require synchronization with the PCI
controller ops, since they are used before the controller registration.

Suggested-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Ilya Lipnitskiy <ilya.lipnitskiy@gmail.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>