rt2x00: Add register definition for busy time on secondary channel

Add the register definition CH_BUSY_STA_SEC for reading the busy time
on the secondary channel in HT40 mode. Also update the comments about
channel busy/idle time registers to express the used unit.

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>