b43: Use 64bit atomic register access for TSF

On modern b43 devices with core rev >=3, the hardware guarantees us an
atomic 64bit read/write of the TSF, if we access the lower 32bits first.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>