rt2x00: Sync Tx and RX ring sizes with legacy drivers

All rt2x00 devices used the same Tx and Rx ring size (24 entries) till
now. Newer devices (like rt2800) can however make use of a larger TX and
RX ring due to 11n capabilities (AMPDUs of size 64 for example).

Hence, bring rt2x00 in sync with the legacy drivers and use the same TX
and RX ring sizes. Also remove the global defines RX_ENTRIES, TX_ENTRIES,
BEACON_ENTRIES and ATIM_ENTRIES and use per driver values.

That is 24 entries for rt2400pci, 32 entries for rt2500pci, rt2500usb,
rt61pci and rt73usb and 128 (RX) and 64 (TX) for rt2800pci and rt2800usb.

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>