rt2x00: Add TXOP_CTRL_CFG register definition

Remove the magic value initialisation of the TXOP_CTRL_CFG register by
defining its fields and using them during intialisation. The field
RESERVED_TRUN_EN is referred to as reserved, however it is set to 1 by
the legacy drivers. Hence, do the same.

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>