rt2800usb: add RXINFO_DESC_SIZE definition

Add RXINFO_DESC_SIZE definition and use it instead of abusing
RXD_DESC_SIZE one (TXD_DESC_SIZE and RXD_DESC_SIZE are specific
to PCI version of the chipset).

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>