rt2x00: Optimize register access in rt2800pci

All register reads/writes in rt2800pci were previously done with
rt2800_register_read/rt2800_register_write. These however indirectly
call rt2x00pci_register_read/rt2x00pci_register_write which adds an
additional overhead of at least one call and several move instructions
to each register access.

Replacing the calls to rt2800_register_read/rt2800_register_write with
direct calls to rt2x00pci_register_read/rt2x00pci_register_write gets
rid of quite a number of instructions in the drivers hotpaths (IRQ
handling and txdone handling).

For consistency replace all references to rt2800_register_read/write
with the rt2x00pci_register_read/write variants.
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>