rt2800pci: fix comment about register access

Registers used for indirect BBP and RF registers access are
respectively BBPCSR and RFCSR, also make it clear that all CSR
registers access goes through rt2x00pci_register_[read,write]()
methods.

Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>