rt2x00: Centralize setting of extra TX headroom requested by rt2x00.

rt2x00: Centralize setting of extra TX headroom requested by rt2x00.

commit e6218cc47bd54710dc523e8c983ceddba625e3ae upstream.

Set the value of extra_tx_headroom in a central place, rather than in each
of the drivers. This is preparatory for taking alignment space into account
in the TX headroom requested by rt2x00.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
[bwh: Adjust for 2.6.32]
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>