rt2x00: provide separate information about TXWI & RXWI sizes

On new 2800 hardware sizes of TXWI & RXIW can be different than TXD
& RXD sizes, so we need to difference between them. Let's define
winfo_size as size of in buffer descriptor (TXWI & RXWI), and desc_size
of as size of additional descriptor - in separate DMA coherent buffer
for PCI hardware (TXD & RXD) and yet another in buffer descriptor for
USB hardware (TXINFO & RXINFO).

Change is rt2x00 wild, but should affect only 2800 driver.

Patch also fix beaconing for 5592usb AP mode.

Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>