b43: HT-PHY: implement stopping sample tone playback

It was another sequence I recognized in HT-PHY dump:
 phy_read(0x00c7) -> 0x0001
 phy_read(0x00c3) -> 0x0000
phy_write(0x00c3) <- 0x0002
 phy_read(0x00c3) -> 0x0000
phy_write(0x00c3) <- 0x0000
The difference to N-PHY is that it writes to 6 tables instead of a one
(after above).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>