rt2x00: move extra_tx_headroom field from rt2x00_ops to rt2x00_dev

The extra_tx_headroom field of struct rt2x00_ops
indicates the extra TX headroom size required for
a given device. This data is redundant, the value
can be computed from the desc_size and winfo_size
fields of the TX queues.

Move the extra_tx_headroom field to struct rt2x00_dev,
compute its value in the probe routine and use the
cached value in the rest of the code.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Stanislaw Gruszka <sgruszka@redhat.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>