rt2x00: fix rt2x00pci_register_multi[read,write]() arguments

Change type of 'length' argument from u16 to u32 (all arguments
match rt2x00usb_register_multi[read,write]() ones now).

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>